hook_privatemsg_block_message runs twice

I didn't see an issue for this anywhere. Sorry if this is a repost.

It looks like hook_privatemsg_block_message always gets called twice. I have a custom module implementing this hook.

When I try to reply to a message where the recipient is blocked, I get the error message (in the list) displayed twice. When I dpm in my hook, it shows it running twice. Attaching a screen shot.