Aha, thanks for getting back to us on the solution you found. Yes, trying to add a list of discount codes that contain any duplicate(s) of existing codes applied to the same thing would cause a failure to add the discount. That scenario should display an error explaining the problem, rather than falsely saying the discount was added successfully, so I've reported that to Development for correction.
BTW, there is also a limit on the total number of discounts an account can have: 10,000 max. If you reach that limit, we can have Dev remove any expired discounts from your account and/or enable a routine that removes discounts automatically as soon as they expire.