No, TUs are not added to multiple TMs. Firstly, according to Scenario 2 in the document attached to the message I sent on 14 Dec, duplicate TUs appear even when only ONE memory is used for the project (but through the Any TM option). In other cases, I may use multiple TMs, but only one is usually updated and the rest are only used for look-up. In all cases, I have checked and duplicate TUs appear in a single TM (NOT in all of them). Otherwise, why call them "duplicates"? I have done some research of my own online and other people seem to be having the same problem as well. I have also discussed the issue with colleagues over the last week and nearly all of them have encountered this problem at one stage or another, although they are not sure if it happens when they use the Any TM option.
And again, just to be clear, we are talking about duplicate TUs in a single TM each time. This means translating a TU, saving it, going back and making a correction to it, saving it again, and having two versions in the SAME TM (the older incorrect one and the updated one). Actually, I was playing around just now and I repeated the process 6 times (correct a segment, save it, correct it again, save it again, etc.) and ended up with 6 TUs in a single TM for a single segment! Is it possible for you to recreate Scenario 2 using the Studio build I am using and see if you encounter the same problem?
Lastly, I do not inadvertently use the "Add as New Translation Unit".