MP4 reader emits warning if multiple but identical FourCCs are present

I've got a warning when multiplexing an AV1 file. The file play well, but not sure what the warning means.

--- Warnings emitted by job 'Multiplexing to file "****" in directory "****"' started on 2024-10-17 12:28:43 -04:00 ---
Quicktime/MP4 reader: Track ID 0 has more than one FourCC. Only using the first one (0x61763031 "av01": AV1) and not this one (0x61763031 "av01": AV1).