Why are there 2 .mov files for some videos in my Photos library?

Hi folks. I'm working on reorganizing and cleaning up my Photos library, specifically the videos. I exported all of them to 1080p (not using the Unmodified setting because I wanted to convert the 4k to 1080 to save space) and I notice tons of the videos have a filename.mov and also a filename.mov (1) file. The file with the (1) suffix is a bit smaller than the file without the suffix. I want to know what the purpose of this is and how to stop this from happening in the future. Thanks.