Was it a mercenary card or a portrait?
Regardless, I see no issue yet. See this article: https://eu.battle.net/support/en/article/32545 .
Since reading doesn’t seem to be a forte of many forum participants (modern educayshun, I guess?), let me underline a key part for you: there is only one item (out of five) in each pack that has got to do with duplicate protection and so on, the rest can give you anything — even, for example, golden portraits for legendary mercs that you own, even if some are still missing in your collection.
For further examples and illustrations, you can also watch some of these:
https://www.youtube.com/watch?v=DlsYJI5cHB0
https://www.youtube.com/watch?v=mrCEyuQBCSo