I just installed the AvHistory 3us B-25H.  Every time I start CFS3 a message comes up thet says "UnitData Model Name 3us_b25h already in use!"  The program still runs fine, but I can't use the aircraft.

My is fine. Go to aviation History, click on message board and post on aircraft sqwaks about this. When you download their aircraft you clicked an agreement you would take any problems to that site so they can help. Its their plane and they have the answers.
Wulfmann

i solved my problem.  For some reason there was a copy of the aircraft in the weapons folder. ???

That happens to us all. Be careful when you are putting a folder in a folder that has other folders. If you inadvertanly drop it on another folder instead of an empty area, it can go inside the unintended folder .
Example (cause it sounds confusing to me and I am writing it!) Say you are putting the 3gr_bf-109g_6 into the aircraft folder of CFS3. but, as you go to drop it, it lands on the Do-335. You close it and can't find it in the game. You look for it, but its not there. You can't imagine what happened and you download and try to install it again. But, the thing ask if you want to replace it cause its there. WHERE!!!!!????. So you give in and say yes, but you still can't find it. What do you do. I looked in every frigging aircraft folder until I find it and bingo, I learnt me dem dar lesson! We take our eye of it at the last second and drop it wrong. You have seen athletes do the same thing.
Wulfmann