FCPX relinked to wrong file, won't let me relink to correct one

Sorry about the problem. In general, each filename should be globally unique. That means unique across the project and across all projects you ever do. This is especially a problem with Sony mirrorless cameras which restart filenames with C0001.mp4 each time a card is inserted.

If the particular camera permits, set it to not restart filenames on new cards, and apply a filename prefix unique to that camera.

You can achieve unique filenames by appending a numeric incrementing suffix to each file before import, then upon the next import start with +1 from the last number. The Mac Finder can do this, see available tutorials on Finder rename procedures.

If you ensure unique filenames it will make both post production and archival file management smoother. It will also prevent various anomalous cases like you are now experiencing.

The Sony mirrorless cameras put the media files in a folder tree which can complicate renaming, or importing using "leave files in place". One solution is rewrapping them with a product such as EditReady, or you can remove the MP4 files from the folder tree, rename them and import in place. In general it's not considered good practice to remove files from a media folder tree since in theory the sidecar files might have important metadata. However I have done that on thousands of files from Sony A7-series cameras and never had a problem in FCP.

Not sure how this will work, but it may be worth a try:

Open a backup of your library from just before you relinked, and try to relink that to the correct files.