My girlfriend used ella twice in two weeks in one cycle. once because of precum once because i ejaculated in her. would it decrease its effectiveness

Although : emergency contraceptives can be used more than once, repeated use may decrease effectiveness. The two of you need to me more responsible about finding a regular form of birth control.

Morning after use.: In short, multiple uses won't effect effectiveness. I would discuss a better family planning program jointly with your girlfriend and her OB/Gyn or Family Physician.