The continuing existence of kings under a republican form of government

Today I read through an MPhil dissertation on an Asante chieftaincy dispute. The parallel structure of traditional leadership which is monarchical in nature with a national republican government and the coexistence of customary law and legislated law creates an odd dualism. Ghana is a republic and yet it has a lot of kings, queen mothers, and other royal figures. Normally if a country is a republic it has no kings or royal figures.