Okay, I understand now where this is coming from.
Currently, we can buy items for special currency but when we do we define price in itemtemplae with what it cost in the special currency and then it would have problem selling for coin as it should.
However, I don't see how duplicate items helps that problem though so not sure why you think we would save database space on that account. Currently I see duplicate df items but do not t hink they are there for that purpose, seems like created new items instead of updating to quality changes.
I was thinking the price was set on the merchants that sell for special currency. That seems more logical place to put it to me though do see some issue with it, that makes it harder to keep price consistent if it is on multiple merchants but typically they are on one maybe 2 merchants or if common among realms it might be 3 times that, but usually different realms have somewhat different items.
As far as I know, you don't sell these items for the special currency, you only buy so seems to me merchant be the only place they be needed.