Cross-Seeding:
Alot of times when you download the torrent from another tracker instead of hash checking a pop up will appear that says you already have the torrent and do you want to add the trackers to it. Select Yes and your done.

Sometimes you cant cross seed because some trackers will add something to the filename or the folder filename so it wont work. Hd-mkv is the worst offender that immediately comes to mind. If anyone knows a way around this id like to know.