You can easily accidentally submit a comment twice if the form submit request loads slowly. This is easily fixable with javascript but not as easy without it.
Suggested approach: issue a comment token when you load the page and stick it in the form. Stick that token in redis on submission to prevent dupes.
Perhaps easier is to have a set of hash(user ID + thread + message) and check if it's unique. Can't see why you would want to post the same message twice in the same thread.