While this entry says that Milka is a variant of Milica (which is perfectly true in a general sense) there is no implied 'priority' between these names, and not much else is conveyed to the reader. Both come from the root mil/a/, but then they use different feminine diminutive suffixes: Milka= Mil/a/ + ka; Milica= Mil/a/ + ica. Also, in Croatia Milka was the everyday short form of Ljudmila; if she was officially and properly named Ljudmila, she was usually called Milka. [noted -ed]
Milka (Милка) is very common Bulgarian name also. There are more than 20000 Bulgarian ladies named Милка. I'm one of them :) [noted -ed]As far as understood name can be found as well at Bosnia and Herzegovina, Turkey.
https://en.wiktionary.org/wiki/%D0%9C%D0%B8%D0%BB%D0%BA%D0%B0#Macedonian [noted -ed]