Stopping duplicate watchers?

Hello all,
I’m on RT 4.4.1. Someone just pointed out an interesting problem to me, and I’m not sure how to solve it. Often, someone who is permanently CC-ed in a ticket will appear as an option for one-time CC. Checking them will make them appear on the ticket twice, which also makes them get two emails for the update you submit. Apparently, this happens often enough within our company that people wonder if there’s a way to fix it.

My first thought is a way to remove watchers from one-time CC if they also appear on CC. However, this seems like an obvious thing to do, and something that RT should already be doing. Instead of me trying to cobble together a script with the sad bits of Perl I sort of know, is there a setting or feature in RT itself I could use? Did I just miss the step that can take care of not duplicating notifications to users? Thanks in advance for any ideas.