tq calls the wrong callback in the some situations.  The function
tq-process-buffer contains the following snippet.

(unwind-protect
    (condition-case nil
        (funcall (tq-queue-head-fn tq)
                 (tq-queue-head-closure tq)
                 answer)
      (error nil))
  (tq-queue-pop tq))

tq-queue-pop removes the top question and sends the next one.  But here
a problem occurs, if the callback function adds another question, sends
it and waits for it's result.  Because this invokes tq-process-buffer
recursively with the first question still at top.  Resulting in the
callback to the first question called a second time, third time ... .

Here is a lisp file which should demonstrate this.