Wa alaykum assalamu warahmatullah wabarakatuh

In the Name of Allah, the All-Merciful, the Mercy-Giving

All praise is due to Allah. Peace and blessings be upon his Messenger.

This is a controversial issue indeed. Juristic opinions in this regard could be summarized as follows:

1- One has to perform ghusl again. This case, according to this opinion, is analogous to the case of a person who has performed wudu after urinating then urine comes out after finishing wudu.

2- If the discharged semen comes out without sexual pleasure, this means that it is only the remnant of the previous discharge and thus one does not have to perform ghusl again. If with sexual pleasure, however, one has to perform ghusl.

3- If one has urinated after the first discharge of semen, then this second discharged is likely the result of a new sexual pleasure, which requires new ghusl. Otherwise, one does not have to perform ghusl again.

The second opinion sounds more reasonable. However, one may prefer to take the safe side by following the first one.

Allah Almighty knows best.