Duplicate TUs created when making corrections in Editor mode

I know that this issue was raised a couple of years ago here but it doesn't seem to have been resolved and I'm not sure it's even the same problem as the one I have been having for a while having.

Here goes: I have started a project as "Translate single document" and I have created a TM specifically for that document. After translating a segment, I confirm it, it updates my specific TM, and I move on to the next segment. Upon completing the document, I go back to the beginning and revise what I have done. If I make a correction, and then confirm the corrected segment, I end up with two TUs: one with the corrected text (with CM in green next to it) and another with the old text with a 100% match. I would have thought that the old text would be overwritten.

What am I doing wrong? Any ideas?

 

Thanks in advance