Here is a story to explain my worries.
MR A donates his sperm (which goes to MRS B). At the same time or sometime after MR A has sexual relations with MRS A and produces MASTER A (in this time MRS B produces MISS B). One day MASTER A and MISS B meet each other, have sexual relations and produces…. oh good lord inbreeding has occurred AAAAAGH!!!
Just a thought and highly unlikely… but still possible!