Mutation?

It could very well be a mutation. Usually the first set of leaves after the cotyledon are single-bladed. Don't fret, mutant weed is awesome. Had my fair share of them, and they performed just as well as their normal counterparts. Albeit odd-looking, it didn't impact growth negatively IME. Unless she's runting out and growing far slower than the rest of your plants, just let her do her thing.