Not even the interest is double taxed.
Let's say on 12/31/2019 your account balance is $50,000. During 2020 your account experiences a rate of return of 5% and there are no contributions. Your balance on 12/31/2020 is $52,500 and you immediately take a distribution. Your taxable salary for 2020 is $100,000 so your total taxable income for 2020 is $152,500.
Now let's say you take a loan on 12/31/2019 for $10,000 at 5% interest, payable as single installment in 1 year (ignoring the quarterly requirement for simplicity). On 12/31/2020 your account is $42,000, plus you make your single loan repayment of $10,500, then take your distribution of $52,500. Once again your taxable income for 2020 is $152,500 - exactly the same as without the loan.
If the interest portion of the loan repayment were pre-tax, then the taxable salary would have been only $99,500 and the total taxable income would be only $152,000 - less than without the loan.