But then why would you experience the same problem on a completely different bike that happened to share the same gearing as your bike? To me that points to improper shifting technique. I also see no discussion of looking at the front shifter to see if it is indeed a double instead of a triple, and check to see that there is no fault in the shifter (slipping cable, gunk in the mechanism, etc).

You need a find a mechanic who is interested in actually solving the problem, rather than telling you that there is a problem and then shrugging. If that means swapping out parts until the problem is solved, then so be it.