I wouldn't say it was really common but it does happen and can with most any thing grown from seed. Often it is just what is called a "fused stem" or two seeds that were stuck firmly together. But other times they appear normal except for the extra leaves.

I have seen it with tomatoes and several different kings of flowers. I have read some comments claiming such plants should be destroyed to maintain purity but I never have done so and found the resulting plants to be essentially normal. Let us know how this one does.

Good pics by the way. :)

Dave

Interesting! Maybe it will grow into a monster basil!? I have a few snapdragon came up with 3 cotyledons but nothing like this.