Sunflower with multiple flowers, is this normal?

It's normal for many kinds of sunflowers. It's not normal for others (such as Mammoth Gray Stripe). However, as benn pointed out, it could be the result of cross-pollination (that's my guess, if you have some of both types from the same packet).

We had a cross between Mammoth Gray Stripe and a smaller sunflower before, and the first generation was like that. The subsequent generations were pretty much all like the smaller flowered parent of the cross.

Some varieties of Helianthus have multiple heads, like the variety Valentine for example. I don't know what variety you have there and where the seeds came from. They look more like a giant kind of variety (American giant?). Since they come from the same package (your one headed and your multiple headed plant), it could be that your multi head is a hybrid with another variety that has multiple heads. Accidental hybrid if you bought seeds from a professional breeder, maybe cross pollinated by another variety that was growing near by.

You can try to cut off the buds, and keep only one of the larger ones, and hope all energy will go into that one (no guarantees). Or just enjoy the multiple flowers it will produce!