What do you do with the Sims with the same name?

First names can stay, having only sims with unique first names would be unrealistic.

When I have several sims with the same surname that's realistic too, but in this case I sometimes feel the need to make one of those take the partner's name upon marriage. What can mess with my usual rule of sims taking the surname of the partner that's higher in the pecking order (played sims > townies and "upper class" sims > "nobodys").