You are repaying the loan principal with the pre-tax funds that you were loaned. You are not repaying it with after-tax money. The pre-tax funds in your pocket don't become after-tax funds unless you default on the loan and the loan becomes a deemed distribution. Cash is fungible, so it doesn't matter how you move your cash around outside of the 401(k). Moving cash around outside of your 401(k) doesn't change the pre-tax money into after-tax money. (The money taken from your paycheck is money taken from your pocket, so it is effectively coming from the remaining pre-tax money in your pocket.)
A deemed distribution does not satisfy the loan. If you default on the loan causing the pre-tax funds in your pocket to become after-tax funds, your subsequent repayments of the loan become after-tax basis in the 401(k) so that they won't be taxed again upon the funds being distributed from the 401(k) via an ordinary distribution.
Only the interest paid on the loan gets taxed twice.
Another way to look at it: When you received the loan, you received an allocation of pre-tax money. With each repayment, no matter where the cash comes from, you are returning to the 401(k) a portion of that pre-tax allocation.
