## TL;DR
The agent compared the redline against v2 instead of v3, so every change between v2 and v3 showed up twice. Keep a version registry with checksums, always diff against the newest mutually agreed version, and add a guard: if the change count is wildly above the counterparty's stated changes, stop and recheck the baseline before reporting.

## The symptom
```text
agent diffed against the wrong baseline  -  it compared the redline to v2 instead of v3 and every real change showed twice
```

## Steps
### 1. Build a version registry
Record every version received or produced: version label, timestamp, source, and a checksum (sha256) of the file. v2 and v3 must be distinguishable even if the filenames are similar.
Expected: v2 and v3 registered as distinct versions with different checksums.

### 2. Identify the correct baseline
The baseline is the latest version both sides confirmed (usually the counterparty's previous round). Confirm v3 is the right baseline for this diff, not v2.
Expected: Baseline explicitly set to v3 with its checksum.

### 3. Re-run the diff against v3
Diff the redline against the v3 file. The duplicated changes collapse: each real edit appears once.
Expected: Change count roughly halves to the true number.

### 4. Add the count-sanity guard
Before reporting, compare the diff count to the counterparty's stated change count. If the diff shows 2x or more with no explanation, halt and re-verify the baseline pair instead of publishing the report.
Expected: Stale-baseline diffs get caught before they reach a reviewer.

## When to use
- Each change appears twice in the diff
- The change count is roughly double what the counterparty described
- Filenames are similar (v2/v3, draft/final) and easy to mix up

## When not to use
- The edits genuinely are duplicated in the document (rare; verify)
- There is no agreed version history (first exchange: establish it)
- The duplication comes from text-layer plus OCR double extraction (different fix)

## Why it happens
Version files accumulate with unhelpful names, and the agent picks the baseline by recency or filename guess. Diffing N against v2 instead of v3 replays the entire v2-to-v3 delta on top of the real changes. Checksums and an explicit registry remove the guesswork.

## Edge cases
- Counterparty sends 'final_v2' after 'v3': trust the registry order, not the filename.
- Same content, different checksum (metadata-only change): note it and move on.
- Branching versions (two counterparty drafts in flight): diff each against the baseline separately.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_lIx5iD0FVF6NM4q0a6o50w