Three days ago the same happened to me: I bought a new PC, installed PM5 and when I tried to activate I was told I had already 3 seats on a one seat license!! In France (yes I am a frog) a seat is where you put your butts on, but Jamie Brown explained it all to me
I investigated my old PCs and found I had in fact 4 installations, one on a sub notebook, installed just "to see if it worked" (it din't, too slow on a a too small screen, never used it) and three on the old desktop: the one I used, an old build I kept because it was able to write to the RW2 raw files, and one I suppose originating from a new HD installation.
I contacted CamaraBits sales, was told to uninstall. Which I did, from the notebook and the desktop, then was told a new slot was open for me. So I tried to activate PM on the new PC: didn't work. I was not happy at all because I had really ditched everything. But was saved by Bob Russel: one has to run PM as administrator for the activation process, which worked just fine.
I think it is very easy to over install in good faith, there is no warning what so ever. CB should include a warning and a "seat" count in the activation dialog.
I have now PM running on my new PC and many thanks to Jamie and Bob for patience and expertise.
Francis