I replied to Diamond Tim B in a comment @TimB.
Which Tim B did I just ping? Both?
The one who posted most recently. See also: How do comment @replies work?:
Matching is performed in reverse chronological order, so if five people named John are participating, @john will match the most recent John.
In your case you have pinged diamond Tim.
There doesn't seem to be a way to reply to someone who wrote a comment earlier and has the same username as someone who posted later - there is no way to ping not-diamond Tim right now, unless he decides to write another comment and thereby making the diamond Tim unavailable for pinging under your question.
After some tests in the chat with AndyD273 pinging Secret and Secespitus at the same time by using "@Sec" the mechanism seems to work differently in the chat. See also Disable three-letter pings from chat to save Oliver, Olivier and Olive from Oli's pings
We have to fix this. More accurately, I have to fix this, I'm the Johnny come lately. What do I need to do here? Is it as simple as changing my name in the profile? Will that muck up all the other references in answers that refer back to Tim B UnDiamond?
Hopefully this solves the problem. :)