I am subscribed to the
logic tag in Philosophy.SE, and have set it up to get emails every three hours with new questions.
I also happened to be on the site just now at the same time another user asked a question related to logic, and so I naturally read and answered it.
However, I fully expect that sometime within the next three hours I will receive an email alert letting me know that this same question has been posted - I understand that some people may want this functionality (though why I cannot think), but would it be possible to at least have the option to only have emails sent out for new questions posted under a given tag if they have not already been viewed (or if not viewed then perhaps answered, commented or voted on)? Otherwise the signal-to-noise ratio in the emails sent by the site essentially diminishes.
EDIT: And just to follow up, I did in fact get an email for that question that I had already replied to.