addListener does not check for duplication

[uwydoc]

If one callback is bound to one event multiple times, multiple instances of the callback would be stored, and unbind would only remove only one instance instead of all, which is not what the api describes.
It should be fixed ASAP.

[JacksonTian]

callback can be bound to on event multiple time. I fixed the unbind issue.

[uwydoc]

so quick. 👍 i had thought to push and submit a merge request later today(hesitated cause only one line of code). never mind.

[JacksonTian]

Thanks.