Explaining the setup:
So I work as an editor for a Youtuber and for convenience he has now changed his setup. He used to film with his camera straight to memory card and send me that + whatever screen record + external audio and I would sync it all up and then edit. However now he has changed to recording the camera and screen record next to each other in an ultra-wide with OBS. So basically what I get from him is a 3840x1080 (two full HD clips next to each other) file with the correct audio and everything lined up. It was never that much of a hassle for me to manually sync but for him it's easier to record like this cause it's only one file to send to me and also he doesn't have to climb behind his desk to pop out the memory card each time.
We've used this setup in 25fps for some time and it's worked perfectly fine. However for some reasons that I don't care to explain right now we have now switched to recording in 29,97fps.
HERE'S THE PROBLEM:
Half the time final cut doesn't recognise the clip as 29,97 but as 30 which results in double frames every other second or so. When I force the clip into a project with manually set frame rate to 29,97 I still get double frames here and there. Sometimes when I convert the imported footage into optimized media the double frames disappear sometimes not. If I import THE SAME CLIP into premiere it runs perfectly fine with no double frames. I don't want to switch to premiere I love final cut but this is making me go crazy.
We've tried changing the OBS to capture in 30 fps instead but the camera itself records only in 29,97 so that doesn't work either.
Open the clip in Quicktime and see what frame rate it reports in its Movie Inspector.
Some recording programs actually have a variable frame rate to maintain smooth motion. They do this to avoid the skips, dropped frames would cause.
An NLE such as FCPX might compensate by duplicating some frames if it's slightly lower than the timeline frame rate.