It is one of the few rules in the game with no exceptions and no way around it, so it is worth understanding rather than testing.
What a second attempt looks like
Click a code you have already collected and DoubleDown will tell you that it looks like you have already used the promo code or it has expired.
That message covers two situations at once, which is why it confuses people. Either those chips already reached your balance at some point, or the code has run out its life. Both endings are the same for you: nothing more is coming from that code.
The link is not broken. There is nothing to fix and nothing to report.
Nobody can reset one
We are asked this often, and the honest answer is that no one can undo a redemption - not us, not another code site, and not a friend with a special link.
We are a fan site. We share codes; we do not create them, and we have no way to see inside your game account or change anything in it. If you are truly certain you never had a particular code, the only people who can look at your account are DoubleDown themselves, through a support ticket.
One code, one account
The rule follows the account, not the device or the website. Collect a code on your phone and you cannot collect it again on your computer, because it is the same account either way.
By the same token, two people playing their own separate accounts each get their own single redemption of the same code. That is why a code posted publicly is genuinely useful to thousands of players at once, even though each of them can only use it a single time.
What this means for your routine
Because a code has exactly one chance to help you, order matters more than repetition. Work down from the newest codes, take each one once, and move on the moment you see the already-used message.
There is no benefit in going back over yesterday's list hoping for a second helping - but there is a brand new list waiting for you every day at https://www.ddpcshares.com/double_down_c ... tion_codes, and those codes have never met your account before.