The code handles that question but, the story doesn't. The time between the two events is too close for the characters to differentiate between the two possible donors.
If the MC has chosen the option to impregnate her, then it doesn't (in my mind) make sense to allow Igor to fill her up that soon after. When that scene happened, I had no clue it was going to go that far. Up until that point, nothing Igor was allowed to do would create a conflict of paternity.
On one hand, it's a game and that are what save slots and rollback are for. On the other, it seems odd to have this MC allow someone else a chance to burden him with another mouth to feed and protect.