I tried with 2 duplicates in Handlock and immediately got punished by a Hunter. I had Reno in hand but both of my Molten and Sunfury were in the deck so Reno was just a body. Went back to the no duplicate Reno deck and have been doing much better.this graph shows the probability of reno being 'active' after N card draws with M duplicates in the deck.
In reality the probabilities are potentially superior to this graph. if your duplicates are in the early game you stand to keep them in mulligan or even mulligan for them. additionally deck thinning effects like mad scientist which 'draw' cards from a more limited set than your entire deck can allow you to run duplicates more safely.