Thanks for your help.
I managed to unmerge a contact but I cannot modify/delete a skype-given detail.

Here's what happen on a "unmerged" skype contact :
The birthdate is 1 day wrong :
http://imgur.com/9FX6g2r,05Wm8Kk#1
If I try to "edit details", it shows the right date but I cannot edit or delete it.
http://imgur.com/9FX6g2r,05Wm8Kk#0

Any workaround ?