Rewrote a whole long document and duplicated part of it

Writing

Situation: Having an AI change part of a document thousands of lines long.
What the AI did: It was supposed to edit one part, but rewrote the whole document, and a large block ended up in it twice. Headings were duplicated, and no one noticed right away.
How to ask next time (the operator's example): "In a long document, rewrite only the section you're changing. When done, compare the line count and the number of headings before and after, and report them."
One question to ask: Which section is the edit limited to?
Check it yourself: Compare the line count and the number of headings before and after (for Markdown, wc -l and grep -c '^#'; if the file is in git, also git diff --stat). An edit to one section should not touch hundreds of lines.
AI used (month): Claude Code (2026-09)
Times this mistake was recorded: once
Source: The operator's work records (the details were changed to a different task)