From the Super Mario Wiki
An illustration of GLOM

GLOM (Giant Likeness Outputting Machine) is a device featured in the Nintendo Adventure Book Double Trouble. Built over the course of several weeks by Iggy Koopa, GLOM can use sand to make perfect clones of anything it sees, even images broadcast by the spy camera flowers planted by Iggy throughout the Mushroom Kingdom. An intimidating machine, GLOM is ten-feet tall, moves through a combination of wheels, tank treads and mechanical legs, and is covered in electronic eyes, telescoping arms, and miniature television sets, all of which are inexplicably showing old monster movies. When it moves, the machine leaves a trail of slime, presumably some form of waste created by the cloning process.

When Mario finds Iggy in his Fort Koopa laboratory in the Koopahari Desert, the Koopaling summons GLOM to attack him. As the machine advances on him, flailing its arms, Mario can lure it to the hall of mirrors outside the room, where GLOM will create a duplicate of itself after seeing its own reflection. The two GLOMs then make more GLOMs, and continue doing so until the original overloads and blows up, causing an explosion powerful enough to bring down Fort Koopa. With GLOM gone, all the clones it created dissolve back into sand.

In two other possible scenarios, GLOM succeeds in defeating Mario, either by producing a Mario clone that fights the original to the point of exhaustion, or by spawning a group of Iggy clones that overwhelm him through sheer numbers. In either case, the unconscious Mario is taken back to the hall of mirrors rather than resulting in a Game Over.