**Steps to replicate the issue** (include links if applicable):
* Create a page with some text. Make a couple edits.
* Copy paste the contents of the first page to a new second page. Save.
* Make a couple edits.
* Go to Special:MergeHistory and merge the first page into the second page.
* View the history of the second page.
**What happens?**:
* The edit that created the second page has a positive size diff (e.g. +1234 bytes)
{F37137576}
**What should have happened instead?**:
* The edit that created the second page should have a zero size diff (e.g. 0 bytes). This is because no text was added between these two diffs. The wikitext is identical.
**Software version** (skip for WMF-hosted wikis like Wikipedia):
**Other information** (browser name/version, screenshots, etc.):
* The fix is to have Special:MergeHistory recalculate the sizes of every size diff in both page's history after the merge.
* This has likely created a bunch of database corruption. If worth fixing, would probably need a maintenance script that checks the entries in the merge log, then targets those pages for recalculation.