Doubled pawns most commonly arise when a piece is captured toward the center, resulting in two pawns on the same file. In some openings, such as the Nimzo-Indian Defense, White deliberately accepts doubled c-pawns in exchange for the bishop pair and central control, showing that doubled pawns are not always a disadvantage.
The disadvantages of doubled pawns include reduced pawn mobility (the rear pawn is blocked by the front one), inability to create a passed pawn on that file without assistance, and structural weaknesses on adjacent files. In endgames, doubled pawns are usually a clear liability because they function almost as one pawn rather than two.
The advantages of doubled pawns include opening a file for a rook (useful for attacking) and extra pawn control of central squares. When evaluating doubled pawns, consider the specific position: are they in the center where they control important squares, or on the wing where they are simply weak? Context determines whether doubled pawns help or hurt.