are request object ids unique?

Question

Is it safe to store the request object ID returned by facebooks apprequest as a primary key without the username, or is it only unique for every facebook user?

I am confused because of this:

To get the full request ID, concatenate this with a user ID from the to field: 
<request_object_id>_<user_id>

Answer 1

Yes, you cannot send the exact same Request object to the same user twice, so the request ID is unique for that usage of the Requests dialog. Appending the user ID specifically further identifies which request/user pair you're trying to work with.