Good deal to have $5000.

Since it is a duplex, an income property, you need to calculate the return.

The flood insurance premium should be factored in with your calculation.

If it still makes sense, you should buy. I think $706 is nothing comparing with your rent income.

The next buyer will do their math as well. 

To have $5000 credit is a great achievement, which means they already paid you 7 years flood insurance. 

You can sell it after 7 years if you want. But I doubt you will sell after you see the great rent income each month.