Thanks for your interest in Duplicati!
Did you check out this topic, which is basically just a post of all the command line help output?
If you want to essentially turn off Duplicati’s version backups (so only keep the most recent version of a file) then you’ll want to use --keep-versions=1 which basically automatically do the delete command for any version older than the current.
Note that the delete command doesn’t necessarily actually delete the versions from the archive - it just flags them as “outdated”. Once enough outdated (aka deleteable) versions have accumulated (see --threshold, --small-file-size, and --small-file-max-count settings).
If you want to FORCE the cleanup of outdated versions after every backup then you’ll likely want to run the compact command, though you could probably get similar functionality automatically via judicious settings of the threshold and small-file parameters mentioned above.
Note that using Duplicati in this fashion will likely cause a lot of transfers to/from your destination due to all the cleanup being asked of it, so keep that in mind if you’re using a slow or bandwidth costly destination!