Parallel read bug in regards to duplicate references?

[jdillard]

I think I possibly have a similar bug to #12001.

For some background, I had Sphinx project (v6.2.1) that allowed warnings for a period of time, so naturally things got messy, then eventually the warnings were knocked down to zero and -W was implemented. But while builds usually pass, occasionally a batch of old "duplicate label" warnings will randomly crop up in CI, that didn't trigger before. It also isn't possible to trigger them reliably on local builds.

If I switch one of the extensions to "parallel_read_safe": False to force a serial read the "duplicate label" warnings will reliably show up. This leads me to believe there is in fact an issue with the parallel read being done when it comes to references (possibly only duplicates), it is just tricky to recreate. (edit: It is also happening with "Failed to create a cross reference. Any number is not assigned" numref warnings, but oddly enough not at the same time as the "duplicate label" warnings, even if the duplicate labels are causing the numref issue)

Should that issue be re-opened (even without a MWE)? Should I create a new issue (even without a MWE)? Any troubleshooting ideas I can do on my local build to try and get it to reliably happen on parallel builds?