Hi: Thanks, JoeMike. I checked, but it only shows emails, no numbers.
You know, I think it works like the autofill works in Safari just remembering urls. They aren't stored anywhere obvious, but Safari remembers them. It's a mystery and very annoying. I couldn't find anyone on the internet who had the true answer. I was told there could be a link between my iPhone and my Mac with the numbers, but I was able to successfully remove the number from my iPhone but I still have the problem on Mac Messges. Oh well. I wonder if there is somewhere in the library that keeps that info. I used EasyFind but ony found a couple of deleted texts I had to the correct person.
I am not sure where Messages is getting its list from, other than Contacts, but there is another contacts list in Mail > Window > Previous Contacts. You might look to see if the alternate number is there, and if it is delete it.