So you do not know what is double taxed.

You can look at this $50K as two parts of process:

1) you earn 50K, and you put it in 401k as pre-tax money, you take it out when retired. you pay tax on 50k.
2) you get the loan 50k from somewhere, you pay it back with after-tax money. the loan of 50k is not what you earned, you have to earn 75k to pay it. so tax on 75k is right.

you can look at this as you earned 50k from first part, and another 75k from second part. So you shuld be taxed on 125k (50+75k). No double tax as you claimed.