Is double pawn a bad structure?

Please tell me, thanks.

you get an open file cannot say

usually, double pawns are weaker since they control limited space and can't protect each others but in some positions, double pawns are completely fine to play. Mainly in the middlegame when you want to open the g file on the enemy king.

@hydrophilic Usually, it's bad for the pawn structure, but in some cases it can be good to have double pawns, but normally you should avoid to have double pawns. The reason double pawns are bad is that there are 2 pawns of your color in 1 file and the first one blocks the other one. It's also difficult to protect this because you can't put a rook or a queen behind the pawns to protect both, and usually the first pawn will be taken. If you have double isolated pawns, it's even worse. Because then you can't protect the double pawns with other pawns and you will probably lose one pawn, and then you will have an isolated pawn which is also difficult to protect. The opponent will then try to exchange all the pieces to have a better position in the endgame because an isolated pawn can't be protected by other pawns.

Sometimes you want it.

If the square in front of the doubled pawns can be exploited by the opponent by the means of occupying it (thus blockading the doubled pawns) then the doubled pawns are a weakness, otherwise if there is a pawn on the file next to the file that contains doubled pawns that could support the doubled pawns, then they are no longer weak.

In general you should avoid doubled pawns without compensation. Compensation could be an open file, an extra pawn, tempi in the opening...., any kind of iniative....

Triple pawns are worse

I won games with double pawns and even triple pawns. But I also lost games for the same reason. It all depends on the situation. In general, if you are a beginner, try not to have doubled pawns.