Twitter and every other platform enforces a unique name for every account. Kathy's handle is @KathyGriffin and once it is registered, no other account can use it. Everyone on Twitter knows that handles are unique, that the handle shows up both in the URL *and* below the username, and that the username is NOT unique and can be changed to anything at any time. So if you are trying to find the identity of the posting account, you use the handle, not the username. And in any case, identity theft is a specific crime that requires using someone else's credentials to commit fraud. No fraud, no identity theft.