Option to not download same file twice

[15532th]

In situation when space ended but still available for replay, running twspace_dl -U more than once will result in .m4a file being downloaded again and replacing already existing one. If systemd unit with RestartSec=15 is used, new download will be started as soon as old one finishes the entire time space stays available.

Checking if file with given name already exists before starting download would prevent this from happening. yt-dlp also has --download-archive option, which maintains list of id of successful downloads and skips over them if called twice.

[jim60105]

Using docker-compose to monitor user also encountered the same problem. @Ryu1845 I think #64 should be reviewed.