As has been stated we don't. If you give it to us in your preamble and its correct then it goes on the strip and it shouldn't get mentioned by the controller. In fact it greatly reduces the RT load if you do give us the QNH on first contact so please continue doing so.
There is a lot of coordination between delivery and the other positions so several times I have missed the first few seconds of a new aircraft calling for clearance, I may pick up the callsign and destination and not be entirely sure if I heard the QNH. So simply put, if I'm in any doubt that the pilot did not give it, or I did not hear it, then I will give it again and expect a readback. I can't speak for other controllers but for some it may just be force of habit to give it again regardless!
cheers