

If you’re talking about downloading the same file twice, you can also pause the torrent, put the finished file in the folder it would be downloading to (make sure it fully matches, same name) then right click the torrent and select “force recheck” to make your client see it has it already
.mp4 with H264: the most universal, and can be compressed to smaller sizes than you might think. Compatibility and compression will still vary depending if you use AAC or opus audio
.webm with VP9 and opus audio: better compression, not as universal. More open-standards based, maybe best balance of compression and compatibility
.mp4 or webm with AV1 and opus audio: probably best compression, also probably less compatibility than VP9, maybe depends what devices they use - good on new computers / phones / Android based TVs, more iffy on a wider or older range of devives. MP4 maybe friendlier than webm on newish Apple devices