calling complete! on already completed task messes up stats

I'm not sure if this is as expected, but I found this behavior when I was trying stuff in REPL.

``` clojure
> (dq/put! q :test "test task")
> (def t (dq/take! q :test))
> (dq/complete! t)
true
> (get (dq/stats q) "test")
{:in-progress 0, :completed 1, :retried 0, :enqueued 1, :num-slabs 1, :num-active-slabs 1}
> (dq/complete! t)
true
> (get (dq/stats q) "test")
{:in-progress -1, :completed 2, :retried 0, :enqueued 1, :num-slabs 1, :num-active-slabs 1}
```

Is this expected behavior and the caller is responsible of keeping track of task status?
If so, is there possibility of making status function public, so that we can check the status of the task?(currently it's private)