Support calling remove callback multiple times

[nwalters512]

While refactoring some code, I ended up accidentally calling the remove callback multiple times. It took me way too long to figure out why it seemed like my process was hanging: because the callback passed to the remove callback isn't invoked when the remove callback is called multiple times.

I acknowledge that one shouldn't end up calling the remove callback multiple times in normal usage, but it's a big violation of expectations for my own callback to never be called in that situation. I'm hoping you'll accept this patch. Thank you!