How is development of horseshoe kidney? how are duplicities? How are bifidities?

Fused kidneys, first found in pelvis then go up and stop at inferior mesenteric artery Other abnormality is duplicities (double pelvis, double ureters could be uni or bilateral gives many openings to bladder, we also have bifidity (2 ureters from each kidney merge before they reach bladder (1 bladder opening)) All these live