I'm pretty sure this card will see play. Sure, people will play around it just like they play around Mirror Entity but in constructed, most players play pretty solid minions and getting two of them doesn't seem that bad.
So the mage got two fire flies. Or two alley cats. Is that really that bad? What does the hunter do when you play the next secret? Risk giving you a Highmane on the board? Or two in your hand?
Basically, this secret doubles (or triples if you play potion of polymorph) the number of minion secrets they have to play around and most decks only run so many weak minions before they have to start risking their larger more valuable ones.
Is this secret great? Not really. But it's good, and secret mage is powerful enough that it will see play in constructed. In arena, this card is really good. Card draw that guarantees minions? Yup, sounds good to me.
You do realize that dup allowed you to manipulate the board so that you always got a high value proc by not playing any crappy minions right? With this card, your opponent can easily give you something like 2 copies of a 1 or 2 drop, not to mention the way you play around this card is exactly the same as with entity or potion of polymorph, so he will do it.
This is not even taking into account that duplicate, as better as it was, was a very niche card reserved for extreme lategame decks like control and fatigue mage.
tl;dr: bad card, won't see it in constructed. In arena it's about average or below(basically a shittier Arcane Intellect, which is also about average in arena, or maybe SLIGHTLY above).
Really good, it's pretty damn hard to play around unless you have a 1-drop at hand, or a card like Jade Spirits which is garbage for a non-Jade deck. Though it is a value Mage card, and Mage still needs more healing to play such a deck. Will probably see play in Wild.
It also has some interesting applications for delaying key turns for certain control decks, like a turn 9 Alexstrasza.