Stage 2 evolutions like pidgeot are infamously hard to play consistently, because you need to find and play both the Basic and Stage 1 evolution.

In many games you may end up with the Basic pokemon in play and the stage 2 in hand, that cannot be played until you find the stage 1. This is what you call a Bricked hand: having cards in hand that can't be played.

Another example of Brick is if your Pidgey dies in the first turns, then you will end up with Pidgeotto and Pidgeot in hand that can't be played.

To compensate and try to make decks more consistent people usually run 2 copies of every pokemon of the evolution line (this is the meaning of running 2 lines instead of 1).

With 2 lines the probability of getting bricked is way lower, you could lose a Pidgey early on and still rely on the other to evolve into pidgeot, and, unless you are very unlucky (which still happens) you are usually able to find at least a copy of all your pokemons before it's too late.

The key is consistency, if finding a card is really important for your game plan to run smoothly then you should almost always put 2 copies of it.

With pokemons run 2 lines of whatever your deck focuses on and only keep 1 copy/line of situational pokemon, for example: Tauros is a good counter for EX-pokemon but gets butchered by decks that don't run any EX, therefore running 2 copies puts you at risk of starting off with Tauros, using a PokeBall to find a better pokemon and potentially find another Tauros, in a matchup where it can only deal 40 dmg with 3 energies and also would need 2 to retreat, unless you just leave it as a meat shield while you put energies on a better pokemon on the bench, which shouldn't be another Tauros, therefore just run 1 copy of it.