While there is only one Gaius Julius Caesar, his lastname has been used as title by various successors.

Slim Shady is a self invented stage name so there is not really a point in anyone adopting it as their own unless they are comfortable being called a copycat all the time.

Charlemagne actually is short for the French version of Charles the Great so it is a title name combination just as Alexander the Great.

Atilla probably sounds unique to you because it’s an uncommon name in the west.