## TL;DR

The agent merges pair A-B, then later rediscovers A-B as a duplicate and merges again, forever. Keep a ledger of completed merges (pair ids plus the surviving record) and check it before every merge. A merge already done is not a duplicate found.

## Error

```text
(No error; the symptom is the merge log showing the same pair merged dozens of times, or "already merged" errors cycling.)
```

## Steps

1. Stop the loop and inspect the log for the repeated pair. Expected: you confirm the same ids recur.
2. Create the merge ledger: pair key (sorted ids), surviving record id, timestamp. Expected: durable history of what merged.
3. Check the ledger before each merge; skip pairs already merged. Expected: the loop breaks immediately.
4. Fix the redetection: after merging, the survivor should no longer match the loser's old signature. Expected: the pair stops resurfacing.
5. Add a circuit breaker: if the same pair is attempted N times, halt and alert. Expected: loops fail loudly instead of spinning.

## When to use

- Merge jobs repeat the same pairs.
- "already merged" errors appear in a loop.
- An agent's merge count grows without the duplicate count shrinking.

## When not to use

- First-time merge failures (fix the merge itself).
- Legitimate re-merges after unmerging.

## Tool compatibility

- Any merge API; a ledger in a file or database.
- Agent loop guards.

## Variant phrasings

### merging the same pair repeatedly

Ledger and skip.

### already merged errors in a loop

The redetection is broken, not the merge.

## Why it happens

Merge changes the records, but the duplicate detector's view is stale or the survivor still matches the loser's old fingerprint. Without a ledger, the agent cannot tell done from new.

## Edge cases

- Unmerging resurrects the pair legitimately; the ledger needs a tombstone for unmerges.
- Concurrent workers need a shared ledger; local files do not coordinate.
- Ledger keys must be order-independent (sort the ids).

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_mJJgvReV3q1evm4-PzHSZg