Bibliography mixup after melting two documents together

PJH

I copy & pasted two documents together. After generating the bibliography it doesn't melt the authors right together. Like I had at the first file quoted two documents of the same author in the same year (author 200xa, author 200xb). Pasting it in the second file with a third source from the same year already doesn't make it go author 200xa, 200xb and 200xc but just keeps it without a letter. Any suggestions how to fix this?

Rintze

Is the output style in the merged document the same as in the first one?

PJH

Yes, both have the same style setting.

sean

All three documents have exactly the same author? No differing initials or anything at all?

PJH

Yes. Exactly the same author, same year. i even deleted all references of the one without a letter updated it so it disappeared from the bibliography, added it again and there it was still without a letter.
I can nail it now more down: It is NOT working with the Harvard Style, but it IS working with the Chicago style.
Very very strange...