The definition in general terms of SOvereign is supreme or something that has supreme or ultimate powers, it is was commonly used to refer to people in the royalty that rule a place. On the other hand, supreme means to be superior to others or to have a position of power, then the two words have similar general meanings.
Repetition usually tells us that the someone wants a large emphasis on this word: that it has a significant meaning to them, and/or possibly the whole tale itself.