diff --git a/.gitignore b/.gitignore
index 434f74c..36d3a8f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -17,6 +17,8 @@ htmlcov/
 
 # Runtime state files — personal workout/health data, never public.
 screen_locker/log.json
+# ...and the ledger naming which of its entries record the same workout.
+screen_locker/duplicate_credits.json
 # ...including the timestamped backups a correction script leaves behind.
 screen_locker/*.json.bak-*
 screen_locker/sick_history.json
diff --git a/screen_locker/tests/test_annotate_duplicate_credits.py b/screen_locker/tests/test_annotate_duplicate_credits.py
new file mode 100644
index 0000000..6f4ac97
--- /dev/null
+++ b/screen_locker/tests/test_annotate_duplicate_credits.py
@@ -0,0 +1,163 @@
+"""Tests for the duplicate-credit ledger script.
+
+The ledger is documentation, not behaviour -- nothing reads it to decide a
+lock. Its one hard requirement is that it never contradicts
+:func:`~screen_locker._weekly_check.count_day_credits`, because a ledger that
+describes a different rule than the locker uses is worse than no ledger at all.
+That invariant is enforced at runtime by ``check_ledger_agrees`` and pinned
+here.
+"""
+
+from __future__ import annotations
+
+import sys
+from typing import Any, NoReturn
+
+import pytest
+
+from scripts.annotate_duplicate_credits import (
+    build_ledger,
+    check_ledger_agrees,
+)
+
+
+def _raising_exit(code: object = None) -> NoReturn:
+    """Stand in for the real ``sys.exit``.
+
+    Args:
+        code: The exit status, as ``sys.exit`` takes.
+
+    Raises:
+        SystemExit: Always, exactly as ``sys.exit`` does.
+    """
+    raise SystemExit(code)
+
+
+@pytest.fixture
+def restore_sys_exit(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Give this test a ``sys.exit`` that actually exits.
+
+    The autouse ``_block_real_tk_and_exit`` fixture in conftest patches
+    ``screen_locker.screen_lock.sys.exit``. That attribute path resolves to the
+    *global* ``sys`` module object, not a per-module copy, so the patch replaces
+    ``sys.exit`` for every module in the process for the duration of every test
+    -- including this script's. Without this fixture the guard under test is a
+    no-op and the test passes for the wrong reason.
+
+    Args:
+        monkeypatch: pytest's attribute patcher, which undoes this on teardown.
+    """
+    monkeypatch.setattr(sys, "exit", _raising_exit)
+
+
+def _entry(wtype: str, timestamp: str) -> dict[str, Any]:
+    """Build a log entry with a workout_id, as the real writer does.
+
+    Args:
+        wtype: The ``workout_data.type`` value.
+        timestamp: ISO-8601 timestamp.
+
+    Returns:
+        A log entry dict.
+    """
+    return {
+        "timestamp": timestamp,
+        "workout_data": {"type": wtype},
+        "workout_id": f"{wtype}:{timestamp[:10]}",
+    }
+
+
+class TestBuildLedger:
+    """The ledger names exactly the entries that share one credit."""
+
+    def test_stronglifts_pair_is_recorded(self) -> None:
+        """A phone/PC pair on one day becomes one ledger record."""
+        logs = {
+            "2026-08-24": [
+                _entry("pc_workout_verified", "2026-08-24T10:55:29+00:00"),
+                _entry("phone_verified", "2026-08-24T10:55:38+00:00"),
+            ],
+        }
+        ledger = build_ledger(logs)
+        assert list(ledger) == ["2026-08-24"]
+        record = ledger["2026-08-24"][0]
+        assert record["credit"] == "stronglifts"
+        # Earliest timestamp is the canonical entry; the later copy is absorbed.
+        assert record["counted"] == "pc_workout_verified:2026-08-24"
+        assert record["duplicate_of"] == ["phone_verified:2026-08-24"]
+
+    def test_days_without_a_shared_credit_are_absent(self) -> None:
+        """Separate workouts produce no ledger entry at all."""
+        logs = {
+            "2026-08-24": [
+                _entry("phone_verified", "2026-08-24T10:00:00+00:00"),
+                _entry("runnerup_verified", "2026-08-24T19:00:00+00:00"),
+            ],
+            "2026-08-25": [_entry("manual_workout", "2026-08-25T10:00:00+00:00")],
+        }
+        assert build_ledger(logs) == {}
+
+    def test_uncredited_entries_are_ignored(self) -> None:
+        """A skip marker earns no credit, so it can never share one."""
+        logs = {
+            "2026-08-25": [
+                _entry("relaxed_day_skip", "2026-08-25T18:00:00+00:00"),
+                _entry("relaxed_day_skip", "2026-08-25T19:00:00+00:00"),
+            ],
+        }
+        assert build_ledger(logs) == {}
+
+    def test_entry_without_workout_id_falls_back_to_position(self) -> None:
+        """Legacy entries predating workout_id are still nameable."""
+        logs = {
+            "2026-08-24": [
+                {
+                    "timestamp": "2026-08-24T10:00:00+00:00",
+                    "workout_data": {"type": "phone_verified"},
+                },
+                {
+                    "timestamp": "2026-08-24T11:00:00+00:00",
+                    "workout_data": {"type": "pc_workout_verified"},
+                },
+            ],
+        }
+        record = build_ledger(logs)["2026-08-24"][0]
+        assert record["counted"] == "2026-08-24#0"
+        assert record["duplicate_of"] == ["2026-08-24#1"]
+
+
+class TestLedgerAgreesWithTheCreditRule:
+    """The ledger's arithmetic must match count_day_credits exactly."""
+
+    def test_generated_ledger_agrees(self) -> None:
+        """A ledger built from a log always agrees with that log."""
+        logs = {
+            "2026-08-24": [
+                _entry("pc_workout_verified", "2026-08-24T10:55:29+00:00"),
+                _entry("phone_verified", "2026-08-24T10:55:38+00:00"),
+            ],
+            "2026-08-27": [_entry("manual_workout", "2026-08-27T18:45:32+00:00")],
+        }
+        check_ledger_agrees(logs, build_ledger(logs))
+
+    @pytest.mark.usefixtures("restore_sys_exit")
+    def test_over_claiming_ledger_is_refused(self) -> None:
+        """A ledger claiming a duplicate that is not one aborts the script."""
+        logs = {
+            "2026-08-24": [
+                _entry("runnerup_verified", "2026-08-24T07:00:00+00:00"),
+                _entry("runnerup_verified", "2026-08-24T19:00:00+00:00"),
+            ],
+        }
+        hand_written = {
+            "2026-08-24": [
+                {
+                    "credit": "runnerup_verified",
+                    "counted": "runnerup_verified:2026-08-24",
+                    "duplicate_of": ["runnerup_verified:2026-08-24"],
+                    "note": "wrong: two runs in a day are two workouts",
+                },
+            ],
+        }
+        with pytest.raises(SystemExit, match="Ledger disagrees"):
+            check_ledger_agrees(logs, hand_written)
diff --git a/scripts/annotate_duplicate_credits.py b/scripts/annotate_duplicate_credits.py
new file mode 100755
index 0000000..0d0c5b6
--- /dev/null
+++ b/scripts/annotate_duplicate_credits.py
@@ -0,0 +1,250 @@
+#!/usr/bin/env python3
+"""Record which log entries are two recordings of one workout.
+
+Written for the 2026-08-29 outage: a StrongLifts session reaches ``log.json``
+twice -- once as ``phone_verified`` from the phone sync, once as
+``pc_workout_verified`` from the PC session sync -- and both are genuine,
+signed records of the same hour in the gym. ``_weekly_check`` now collapses
+them onto one credit, so the *numbers* are right; this makes the *history* say
+so rather than leaving a reader to notice two entries share a duration.
+
+Annotates, never deletes, for two independent reasons. Deleting is
+self-undoing: every re-ingest path dedups against ``log.json``'s own contents,
+so ``_session_sync`` re-pulls a removed ``pc_workout_verified`` from the CRDT
+store on the next 15-minute ``workout-sync.timer`` tick and re-applies its
+shutdown credit. And it is the repo's stated position on signed data -- see
+``scripts/revoke_sick_day.py``. No entry is modified, so no HMAC is touched.
+
+The ledger is descriptive: nothing reads it to decide a lock. If it ever
+disagrees with :func:`~screen_locker._weekly_check.count_day_credits` the
+script fails rather than writing, because a ledger contradicting the rule it
+documents is worse than no ledger.
+
+Usage:
+    python3 scripts/annotate_duplicate_credits.py            # show the diff
+    python3 scripts/annotate_duplicate_credits.py --apply    # write it
+"""
+
+from __future__ import annotations
+
+import argparse
+from collections import defaultdict
+from datetime import UTC, datetime
+import json
+import logging
+from pathlib import Path
+import shutil
+import sys
+from typing import TypedDict
+
+from screen_locker._log_io import load_workout_log
+from screen_locker._weekly_check import count_day_credits, credit_key
+
+_logger = logging.getLogger(__name__)
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+LOG_FILE = REPO_ROOT / "screen_locker" / "log.json"
+LEDGER_FILE = REPO_ROOT / "screen_locker" / "duplicate_credits.json"
+
+# A credit is "shared" only once a second entry claims it; one entry per
+# credit is the normal case and earns no ledger record.
+_SHARED_BY = 2
+
+_NOTE = (
+    "Two ingestion paths recorded one workout. Both entries are genuine and "
+    "stay in the log; they share a single credit."
+)
+
+
+class CreditRecord(TypedDict):
+    """One credit that more than one log entry recorded.
+
+    Typed rather than a bare dict so ``duplicate_of`` is known to be a list
+    of strings at both use sites -- otherwise ``len()`` and ``join()`` on it
+    need a typing suppression, which this repo bans outright.
+    """
+
+    credit: str
+    counted: str
+    duplicate_of: list[str]
+    note: str
+
+
+def _entry_id(entry: dict, date_str: str, index: int) -> str:
+    """Return a stable identifier for a log entry.
+
+    Args:
+        entry: The log entry.
+        date_str: The date key it is filed under.
+        index: Its position in that date's list, used when the entry predates
+            ``workout_id`` and so has nothing else to name it by.
+
+    Returns:
+        The entry's ``workout_id`` when it has one, else ``<date>#<index>``.
+    """
+    workout_id = entry.get("workout_id")
+    return str(workout_id) if workout_id else f"{date_str}#{index}"
+
+
+def _shared_credits(date_str: str, entries: list[dict]) -> list[CreditRecord]:
+    """Return one record per credit that more than one entry shares.
+
+    Args:
+        date_str: The date key the entries are filed under.
+        entries: That date's log entries.
+
+    Returns:
+        A list of ledger records, empty when every entry earns its own credit.
+    """
+    groups: dict[tuple[str, str], list[tuple[int, dict]]] = defaultdict(list)
+    for index, entry in enumerate(entries):
+        key = credit_key(date_str, index, entry)
+        if key is not None:
+            groups[key].append((index, entry))
+
+    records: list[CreditRecord] = []
+    for key, members in sorted(groups.items()):
+        if len(members) < _SHARED_BY:
+            continue
+        # Earliest timestamp first: the canonical entry is the one that landed
+        # when the workout actually happened, not the copy a later sync filed.
+        ordered = sorted(members, key=lambda m: str(m[1].get("timestamp", "")))
+        records.append(
+            {
+                "credit": key[0],
+                "counted": _entry_id(ordered[0][1], date_str, ordered[0][0]),
+                "duplicate_of": [
+                    _entry_id(entry, date_str, index) for index, entry in ordered[1:]
+                ],
+                "note": _NOTE,
+            }
+        )
+    return records
+
+
+def build_ledger(logs: dict[str, list[dict]]) -> dict[str, list[CreditRecord]]:
+    """Build the full ledger from a loaded workout log.
+
+    Args:
+        logs: The normalised ``{date: [entry, ...]}`` log.
+
+    Returns:
+        A date-keyed ledger holding only the days with a shared credit.
+    """
+    return {
+        date_str: records
+        for date_str, entries in sorted(logs.items())
+        if (records := _shared_credits(date_str, entries))
+    }
+
+
+def check_ledger_agrees(
+    logs: dict[str, list[dict]],
+    ledger: dict[str, list[CreditRecord]],
+) -> None:
+    """Exit unless the ledger's arithmetic matches the canonical credit rule.
+
+    For every day, the entries that earn a credit minus the ones the ledger
+    calls duplicates must equal ``count_day_credits``. If those disagree, the
+    ledger is describing a rule the locker does not use.
+
+    Args:
+        logs: The normalised workout log.
+        ledger: The ledger built from it.
+    """
+    for date_str, entries in logs.items():
+        earning = sum(
+            1
+            for index, entry in enumerate(entries)
+            if credit_key(date_str, index, entry) is not None
+        )
+        duplicates = sum(
+            len(record["duplicate_of"]) for record in ledger.get(date_str, [])
+        )
+        expected = count_day_credits(date_str, entries)
+        if earning - duplicates != expected:
+            sys.exit(
+                f"Ledger disagrees with count_day_credits on {date_str}: "
+                f"{earning} earning - {duplicates} duplicate != {expected}. "
+                "Refusing to write a ledger that contradicts the credit rule."
+            )
+
+
+def _describe(ledger: dict[str, list[CreditRecord]]) -> str:
+    """Render the ledger as a human-readable summary.
+
+    Args:
+        ledger: The ledger to describe.
+
+    Returns:
+        One line per shared credit, or a single line saying there are none.
+    """
+    if not ledger:
+        return "No shared credits found — every entry records its own workout."
+    lines = [f"{sum(len(v) for v in ledger.values())} shared credit(s):"]
+    lines.extend(
+        f"  {date_str}  {record['counted']}  absorbs  "
+        f"{', '.join(record['duplicate_of'])}"
+        for date_str, records in ledger.items()
+        for record in records
+    )
+    return "\n".join(lines)
+
+
+def _write(ledger: dict[str, list[CreditRecord]]) -> None:
+    """Back up any existing ledger and write the new one.
+
+    Args:
+        ledger: The ledger to persist.
+    """
+    if LEDGER_FILE.exists():
+        stamp = datetime.now(tz=UTC).strftime("%Y%m%dT%H%M%SZ")
+        backup = LEDGER_FILE.with_suffix(f".json.bak-{stamp}")
+        shutil.copy2(LEDGER_FILE, backup)
+        _logger.info("Backed up existing ledger to %s", backup)
+    # indent=2 matches write_signed_entry, so both files diff the same way.
+    LEDGER_FILE.write_text(json.dumps(ledger, indent=2) + "\n", encoding="utf-8")
+    _logger.info("Wrote %s", LEDGER_FILE)
+
+
+def main() -> int:
+    """Show, and optionally write, the duplicate-credit ledger.
+
+    Returns:
+        Process exit code.
+    """
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument(
+        "--apply", action="store_true", help="write the ledger (default: dry run)"
+    )
+    args = parser.parse_args()
+
+    handler = logging.StreamHandler(sys.stdout)
+    handler.setFormatter(logging.Formatter("%(message)s"))
+    logging.basicConfig(level=logging.INFO, handlers=[handler])
+
+    logs = load_workout_log(LOG_FILE)
+    if not logs:
+        sys.exit(f"No workout log to annotate at {LOG_FILE}")
+
+    ledger = build_ledger(logs)
+    check_ledger_agrees(logs, ledger)
+    _logger.info("%s", _describe(ledger))
+
+    existing = (
+        json.loads(LEDGER_FILE.read_text(encoding="utf-8"))
+        if LEDGER_FILE.exists()
+        else None
+    )
+    if existing == ledger:
+        _logger.info("Ledger is already up to date; nothing to do.")
+        return 0
+    if not args.apply:
+        _logger.info("Dry run — re-run with --apply to write %s", LEDGER_FILE)
+        return 0
+    _write(ledger)
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())