Some avocados are polyembryonic, meaning each seed has more than one
embryo. It is not that unusual in a few plant species. Some types of
citrus and mango are polyembryonic.
Only one of the sprouts should be a sexual embryo. The others will be
asexual embryos, which will produce a clone of the parent tree.
Sometimes the sexual embryo dies so only asexual embryos remain.
An internet search on polyembryonic avocado will turn up several
webpages that mention polyembryonic seed in avocado.