>Could it be something changed a value of one of the fields later, so it became a duplicate of another record? One thing to check about these duplicates is if there's a missing value in the sequence of one for some other value of the other field (i.e. NOMEMBER value which is duplicate could be a missing value for some other value of NOCLIENT; it was OK while a pair to this other value).

Wow, that's an interesting one. I will also search in the place where we do those REPLACEs.