I'm trying to understand 'Supernumerary kidney' within a medical context. Could you shed some light on it?
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, or from separate metanephric blastemas 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).
Less than a hundred cases are known of one or two supernumerary kidneys.

See also
Supernumerary body part

References
Luciano A. Favorito; Ana Raquel M. Morais (May–June 2012). "Evaluation of supernumerary kidney with fusion using magnetic resonance image". IBJU. 38 (3): 428–429. doi:10.1590/s1677-55382012000300018. PMID 22765860.