Cash on cash return on a duplex we bought

[Sara Kirkpatrick]

Last fall my husband and I bought a duplex for $42,000, making a down payment of $10,000.

Each unit rents for $550/month and we’ve had it occupied all but one unit for two weeks. ($13,200/year)

Taxes are less than $500/year and insurance is at $700/year. 
we’ve had a plumber over once for a clogged drain for $60.

I come up with a cash on cash of 96%?

I don’t usually figure my deals using cash on cash, but that can’t be right, what am I leaving out/where are my figures wrong?