Bug: duplicate OrganizationUser objects are created in some cases

[Dschoordsch]

A newly added test to check that we're not creating duplicated OrganizationUser objects is failing randomly. It appears there are still duplicates created in certain cases.

@Dschoordsch it looks like this produced a flaky test result for me-- failed first test, passed on second try.

https://github.com/ParabolInc/parabol/actions/runs/7278378730/job/19832437203. Should I make an issue for this?

edit: happened again, so likely not a fluke: https://github.com/ParabolInc/parabol/actions/runs/7278798261/attempts/1?pr=9295

Originally posted by @mattkrick in https://github.com/ParabolInc/parabol/issues/9290#issuecomment-1864849230