With cards like Pot of Dualityaround, I would like to suggest all of you to play cards with the same rarity and language.
For example, you are playing Blackwings and you have a Super Rare Icarus Attack in your hand. You use Pot of Duality, revealing a Common Icarus Attack and two other cards. You add the Common Icarus Attack to your hand.
You set Vayu along with both Icarus Attacks.
Your opponent uses MST on your Super Rare Icarus Attack. Now he knows that most likely your other face-down card is the Common Icarus Attack.
This will cause him to play around it, and thus causing a disadvantage to you.
Another example would be with Kalut. If you already have a Common Kalut in your hand and used Pot to get another Rare copy, then your opponent direct attacks you with Spirit Reaper, discarding your Common Kalut. This means he knows that you still have another Rare Kalut in your hand.
If both your Kaluts were Rare, he would be able to know that you had two Kaluts.
There are also many other cards that can allow your opponent to take advantage of the different rarities and language. So, try to keep all your copies of the same card the same.