Is it necessary to repeat ablution if urine is passed while showering with warm water?

The discharge of a drop of urine invalidates Wudu', whether during or after ghusl, provided there is certainty of its discharge. As for mere doubt, it is not given any attention, because Wudu' is established with certainty and is not removed by doubt. This is based on the principle that "certainty is not removed by doubt," derived from the Hadith: "He should not leave (his prayer) until he hears a sound or perceives a smell." If you are certain that a drop of urine exited during ghusl, then Wudu' is incumbent upon you afterward.