Trying to redeem online iTunes Giftcard and says “already redeemed”

My friend gave my son an online iTunes giftcard for his birthday (He is 10 so is part of family sharing).

When trying to redeem from the email my friend sent it via - I click redeem, an then redirected to the iTunes Store, the online voucher code is pre populated (perfect) yet when I click redeem it says “error - this iTunes voucher has already been redeemed”.

ive checked purchase history - nothing, I’ve checked every possible google suggested location on where iTunes balance can be found. I was beginning to think I accidentally redeemed it without realising (bumped the button) but if that were the case wouldn’t I be able to see the iTunes balance ? I have also logged out of all device iTune accounts as suggested to no avail.

does anyone have any suggestions at all?? Please help...