We're going to need more information. What kind of second are you practicing? It sounds like you either aren't keeping the cards in the deck aligned or your cards are sticking together. Are you using a fresh deck? And try varying how tightly you are holding the deck.
If you're doing a strike second deal - practice more. You can regulate how many cards are pulled a bit with the middle and ring fingers of the hand holding the deck but mostly it's about a gentle grip on the deck and allowing the friction of the thumb taking the card to only pull one card.
If you're doing a push off second - practice more. In Expert Card Technique, under the Push Off Double there's excellent advice on how to get a proper "push off" that only gets two cards consistently - but mostly it's practice.
I see... So it turns out it was all a deck thing. I didn't think a move could be so much deck dependant. I was doing it with a pretty worn out bridge size deck. I switched for a better and poker size deck and held lightly as you guys suggested and now the move seems ok.
Just for the heck of it I also tried with a streamline poker deck and the second deal comes out perfectly and almost effortlessly, which is kinda annoying, because those decks are supposedly very bad for magic, acording to reviews, since they're not 100% paper.
No decks are "100% paper". Well, no decks outside extremely cheap novelty decks.
All the decks that we consider "standard" to use in Magic, ie: Cartimundi and USPCC (which are now owned by the same company) are a core of "stock" which is basically glue with thin paper on either side, and then that is covered with a "finish" that is basically textured plastic. I don't know how much variation there is the formulas used for that textured plastic, but some decks definitely feel more slick than others, and of course there's variations of the stock which make the cards feel springier or stiffer.
There's also plastic decks which are, as far as I have ever seen, made of PVC. Some of them have a similar plastic finish as 'paper' cards, which does make them slide over each other a bit better, but you will pretty much always be able to instantly tell the difference between plastic and paper cards.
There's a couple card makers that still varnish their cards, which does definitely feel different than the plastic finish USPCC/Cartimundi use, but if you were buying those you'd know it.
They all are. You gotta be willing to grab a fresh deck and see if that helps, especially when learning a move. Once you learn it you might be able to use a more worn deck, but it frequently helps when first learning it. Try some Aristocrats as well. They are a little more expensive, but not much and sometimes you really want that nice extra slip. They seem to have a much slicker plastic coating.
If the streamline deck works for you, then it's legit, but you need try the usual decks as well. Get some Bikes and use those as a reference point. T11 sells them in the old Rider Back boxes and they are good cards. I imagine the Standards you can get at the grocery store are the same cards, but I haven't checked. Not all decks are the same. Make sure you try different kinds.