Codex suddenly outputs an identical response from a previous turn after completing several instructions

What version of the Codex App are you using (From “About Codex” dialog)?

26.313.41514

What subscription do you have?

Business

What platform is your computer?

Microsoft Windows NT 10.0.26200.0 x64

What issue are you seeing?

I'm encountering a frustrating and recurring bug in very long-running Codex sessions: after processing several instructions successfully, Codex suddenly outputs a response that is identical to one of its earlier replies from several turns ago. This has happened to me three times so far in a single ultra-long conversation thread where I've accumulated several million tokens of total usage.

What steps can reproduce the bug?

1. The session starts normally and runs smoothly for dozens of turns, session ID: 019c7b56-df49-7141-b172-04d41af15c0a.
2. Codex handles each task correctly, often involving tool calls, file edits, or long outputs. 
3. After 20–50+ successful turns (and very high cumulative token count), when I send a brand-new prompt
4. Codex begins the turn normally, often with its planning prefix, executes a series of simple viewing/reading-type instructions. and suddenly outputs a response that is identical to one of its earlier replies.

What is the expected behavior?

_No response_

Additional information

Right after the duplicate occurred, I asked: "Why did you just output an identical response to one from much earlier in our conversation?"
Codex did not recognize or admit any repetition/bug. Instead, it replied along the lines of:
"The tasks involve similar operations (reviewing or presenting academic content), so I applied a consistent expression template for clarity and coherence."
This explanation is incorrect — the output was not a "similar template"; it was a complete, verbatim copy of a prior full response.
I use 5.4 high and this seems tied to extremely long contexts (millions of tokens accumulated over hours/days of continuous work). It never happened in shorter sessions.