You do not need to report anything regarding a 401(k) loan repayment in your tax return.
Indeed, you are repaying the loan with after-tax money. However, the loan distribution was also given to you tax-free. The impact to your taxable income occurred when you originally contributed to the 401(k), and will occur again when you draw on this at retirement (or take early withdrawals that are not loans).
Regarding the interest, yes, you are paying the interest with after-tax money, with no offsetting deduction available. So you'll be double-taxed when you receive this back as income in retirement. However, since you are paying the interest to yourself instead of a bank, most people find this result tolerable.
Here's some more information on 401(k) loans: 401(k) loan
