On the possible exploit: I did not experience this myself when disenchanting cards yesterday so it’s not something across the entire board. You might need to supply more specifics. But please do not do that here. Reports that include details that enables others to cheat or exploit should not be made on a public forum. Instead, mail to hacks@blizzard.com as instructed here:
How to Report Hacks/Exploits/Cheating
For the duplicate protection, do keep in mind that it does not look at what you currently own, but at everything you ever owned even if you disenchanted it later. Once you “have or have had” two of each card of a set/rarity (or one for legendaries), cards are strictly random.
So if you dusted some cards and now expect to get those back because you “miss” them, then it’s not a bug but you misunderstanding the feature.
If the above is not your case, then please share more details: which packs did you open, which cards did you get, and which cards were you expecting to get?