A supernumerary kidney is an additional kidney to the number usually present in an organism. This often develops as the result of splitting of the nephrogenic blastema

Blastema

A blastema is a mass of cells capable of growth and regeneration into organs or body parts. Historically blastema have been thought to be composed of undifferentiated pluripotent cells, but recent research indicates that in some organisms blastema may retain memory of tissue origin...

, or from separate metanephric blastema

Metanephric blastema

The metanephrogenic blastema or metanephric blastema is one of the two embryological structures that give rise to the kidney...

s into which partially or completely reduplicated ureteral stalks enter to form separate capsulated kidneys; in some cases the separation of the reduplicated organ is incomplete (fused supernumerary kidney).