We just had pretty much the same thing today. What was odd, that all other experiments (and new ones) were fine and sync’d ok the second time. It was this one particular experiment that it wasn’t happy with (even after moving the folder, deleting the project + git folders etc).
Every time it kept pushing initially and created a new project, but then on a resync it would pick up an old Gitlab ID from somewhere.
In the end we had to remove the .git folder + gitignore, simply rename the file and the Experiment Name under settings, and everything went fine (with no resyncing issues).
It seems at some point the project had been pushed early on, then deleted on Pavlovia - but in the background not fully deleted (i.e. nothing visible on Gitlab/Pavlovia any longer, but something cached/lingering behind the scenes).
My guess is that PsychoPy/Pavlovia tries to do match on Pavlovia/Gitlab for the experiment name under the logged in account, and if it finds a match, assigns the found Gitlab ID to the project. Renaming the experiment name meant it couldn’t find anything, so kept the Gitlab ID allocated on first sync.
Hope this helps.