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?

Females don't produce eggs. They just provide the womb.

Multiple gametes from multiple males fuse nuclei and then undergo meiosis, on multiple cycles. They then form colonies which are fully grown adult beings in the alien's anamorph stage in their life cycle. Each colony then produces a single diploid egg and dies. The eggs attach to the wall of the womb and are born as children. The children eventually grow to be adult aliens in the teleomorph stage of their life cycle.

Having multiple males impregnate the female is a way to ensure maximum genetic variability.

Finally, the gene editing is done by the anamorph colonies in the female womb. The colonies fight each other and the losers die without leaving an egg behind. This fight does a darwinian selection of genes.

The simplest answer is the one humans already have:

You don't know exactly when you're most fertile

Women typically release one egg at a time. That egg has to travel down a tube to the uterus. That trip is the prime time for insemination to occur. While some women have an indicator that ovulation has happened, most do not. Moreover, the fertility cycle of some women is not regular (ovaries don't know they have a 28-day average). They only see the after-effects of not having become pregnant.

As such, a human woman may need multiple insemination events before she becomes pregnant. The advantage to this is it keeps interested men near her (and thus more likely to help her through the pregnancy and child-rearing). Many animals, by contrast, typically mate only at peak times and the female is left to protect and rear young ones by herself.

The ability to screen, by choice, certain genes would provide a strong pressure to mate with as many potential good partners as possible. As soon as that trait has evolved it’s a short hop to the mother’s body refusing to pass gestational milestones without additional material to ‘choose’ from (even if it’s from the same father, the body doesn’t know that).

As for evolving the ability to screen for gene compatibility, the obvious reason for wanting that is it lets potential mothers consciously pick and choose what genes they want, which could (depending on how well the mothers choose) confer significant evolutionary benefit.

If you assume your mothers are capable of choosing appropriate sets of genes to achieve their desired effects (which is very, very much a nontrivial problem), then the survival rates of their offspring will be higher. Mothers who get more potential genes to choose from can maximise this, thus some women evolve to go into ‘heat’ and be receptive to new material often. If they don’t get it their body assumes the child will be born defective and re-absorbs the nutrients (much like rabbits in overpopulated warrens). Eventually only females who mate multiple times exist, and the ones who can mate just once disappear as they keep on wasting valuable energy on children with defects, severe or otherwise.

All the Eldar have personal immortality and a stake in the shared consciousness of the Eldar species. Their population is maintained at a stable and optimal level according to their science and their traditions.

Any time the sad occasion arises that misadventure removes a body/consciousness from the group, it is only fitting that when the shared consciousness detects a suitably advanced vessle for replacement that the mother containing it will be sanctioned to go ahead and prompt the foetus to grow.

At the moment of birth, a convulsion passes through the shared psychic link, churning all the shared memories of the Eldar-who-passed and thrusting them directly into the consciousness of the newley emerging baby. It contains the seed of it's own being, consciousness and it's own DNA.

That which was gone is reborn.

This mechanism appears to be VERY detrimental to species survival. Over millennia, these species would be at a clear disadvantage against their single-insemination peers.

So, to make it at least somewhat practical, we need two things - Eldars must be very communal and very sexually active. This way, it doesn't matter if female would miss her second or third encounter with the male, or if the original male would die - there would always be an avilable "contingency".