The Eldar are a psychic species with a similar body design to humanity within a galaxy-wide empire. They are far more advanced than other races, technologically and philosophically. Eldar have a strange way of reproducing, which requires multiple inseminations in order for conception to happen.
A male must add his genetic material at specified intervals throughout the year to the developing child. Genetic material could come from a single male or various ones with no connection to each other. This allows the female to psychically screen and edit the genes she has been given, keeping the good ones and disregarding the rest. The resultant offspring is a hodgepodge of various genes from different parents.
The reason for this is because the galaxy is made more dangerous by the effects of the warp. This parallel universe is the sum of all emotions taken to the extreme, and filled witch creatures called daemons. This realm is currently bleeding into reality, and is a very corrupting force that causes mutations in anything it touches. Mutations may immediately show up in a individual, or can hide in their genetic code to affect future generations. This form of reproducing is meant to protect the Eldar as a species and prevent them from introducing infected and chaotic genes into their makeup.
How would a species evolve to reproduce in this way?