This typically indicates one of three things:
A file originally in Matroska ( .mkv ) format that was converted to or wrapped in an MPEG-4 ( .mp4 ) container, with the original extension left in the name for tracking. Bidi 01.mkv.mp4
A manual renaming error where the new extension was added without removing the old one. This typically indicates one of three things: A
This usually denotes a sequence or version number, suggesting this is the first file in a series or the first draft of a project. Potential Origins Bidi 01.mkv.mp4