Under the "Interesting tags" list, there's a search box for adding new tags to the list. This search box offers suggestions as one starts typing in it. When searching for things related to existing selections, the suggestions will tend to start with the tags already chosen, since they are the most popular. Those should be filtered out of the search, as well as tags in entry box about to be added.
I don't really find it that inconvenient. It's not hard when the suggestion list comes up to press the down arrow a couple of times. I think adding this feature is probably more work than it's worth.