Warn the user when posting a link that's already been posted
I couldn't find an issue for this so if there is, feel free to close this one.
When posting a link the user should be warned if that link has already been posted in that group.
Idea by Krael: add a repost tag automatically for duplicates.
Gonna add that this warning should only happen if it's a recent duplicate. If a link was posted a month ago I don't think it should give out the warning. As well as that it should only warn if it was posted in the same group.