I'm not sure if this would be allowed, or how many people even know about this as I found it in the Beta. It's likely the Devs don't know about it for now, as it wasn't patched today.

Back in the Beta I randomly lagged out in-game, and when picking my 5 cards from a checkpoint I ended up just spam-clicking through like normal, but due to the lag it picked the same card 4 times, and it stacked! This still works in the Live version of the game, but without intentionally lagging your game while picking cards requires incredibly fast clicking speed. Using an autoclicker you can pick 3 of a single card, but it takes away your card draws equally.. so if you have 2 draws from starting Act 2-1 you can only pick a single card twice and no more, if you choose to do so. Normally you can get a single duplicate card (2x) without a script/autoclicker, but it is still very precise. I'm not sure if it's possible to do with a controller, however, but can be done by just mashing LMB with a mouse fast enough.

The benefit of this would be stacking something like Mad Dash or Run Like Hell, where you boost your Sprint Speed bonus from 20% to 40% or 15% MS to 30% flat, at the cost of your second card draw. Considering how this works, you wouldn't be able to use it in Act 1, unless you start from a checkpoint. A lot of cards do stack, even in Swarm (pvp) mode. Just about everything with a stat increase works. Things like Experienced EMT which boost the other player's max HP upon heal do not, as it's a buff given. Some trial and error would be required to figure out everything.

Stacking damage cards or movement speed cards would be the best idea here, obviously, but in general you can make some crazy decks with this.

I managed to get 105% damage reduction using: 2x Numb [+15% with Temp. Health] (30%) 2x Motorcycle Helmet [+15%] (60%) Spiky Bits [+10% with Melee] (70%) Padded Suit [+10%] (80%) Hunker Down [+10% while Crouched] (90%) Motorcycle Jacket [+5%] (95%) Holly's Passive [+10%] (105%)

Other notable cards worth stacking would be anything with Sprint Efficiency (Superior Cardio, etc.). As far as I've noticed, absolutely none of the Negative card effects stack, only the positive, except maybe the -5% Damage Reduction, although I'm not certain.

Unlisted Demo video, albeit a little laggy lol:

duplicating cards has been known for a bit and was actually banned 2 days ago due to the possibility of cheating and making it hard for us to tell.