MyBB Hacks

Is there a way to only allow an xthread field to be posted once?

For example, the video gallery. Is there a way to only allow a youtube video link to be posted once, and be disallowed from being posted again?

No such feature exists in XThreads unfortunately.
If you don't mind users getting ugly errors though, you could always stick a unique index on the URL column in the threadfields_data table.