org-set-tags-command allows duplicates. Example:
   - M-x org-set-tags-command -> "tag"
   - M-x org-set-tags-command -> "tag"
   Result:
   * Title    :tag:tag:

   I think it would be handy to prevent this.
   Not unimaginable that someone could use this as a feature, though, in
which case, a flag would be nice.

   Regards,
   Dmitrii.