When I type a project or user name, I expect the dropdown not to suggest things I already added. Otherwise, especially when the list of options is long and only the top 4 are shown, I have no way to add new items.
At HEAD, you can use the new "Browse" dialog to browse through a long list of similarly-named items -- you can try it on this install by clicking the new icon inside the tokenizer control:
I'd hesitate to implement the solution exactly as described because I worry it would be confusing to type an item which you know has matches and get nothing back. This isn't insurmountable, but I think it's significantly more clear to always show the match (ideally, perhaps we would visually indicate that it has already been selected in the dropdown, though).