Income you gave back

[cypress4291]

If I gave income back (due to a customer duplicate payment), but not until the following year, do I enter it in the year I reported the income, or wait and enter it the following year?

[Bsch4477]

Assuming you are on a cash basis you report income in the year received and expenses in the year paid. 

[AmeliesUncle]

I don't think it is income and would not include that or the repayment anywhere.

The money never belonged to you.  Just like a refundable deposit, it does not count as taxable income until it 'belongs' to you.