Some seeds make multiple plants normally. Key Limes and Beets come to mind. Avocados might be the same way.
Remy

I have never seen this but like remy says it is always likely. If you do not like this you can always prune back as many as you do not want. Most of us prune them back so they will bush our for a nice looking plant rather than the straight one trunk plant.

Mine did the same, but one main stem has become clearly dominant. I am going to head it back - partly because I want to graft it to a mature tree, but partly also because I want to encourage the other shoots to grow.

I like my trees bushy, even though long-term the avo limbs will tend to droop all the way to the ground.