Dupe

How to perform
Apart from Main Menu Storage, dupes are caused by touching a transition twice. This can be done in two ways :

Get early control, typically from dropping into a hazard with inventory/map open, then opening and closing inventory. Then go into a transition (usually jumping into it). Transitions remove control, but hazards give you control back after a certain amount of time. Because you gained control early, you hit the transition while the timer is ticking down, and when it is done you will have control again. At that point, hit the transition again to get a duped room. The reason you usually jump is that transitions lock your momentum - when you will gain control again, you will also start falling again - the transition locked jump momentum allows you to make it back to the transition while falling, and not just fall out of bounds.

The second way is only doable on upwards transitions, with ground (relatively) close below. Fall down from the transitions, then jump back up as soon as possible, then jump again. The transition has a timer to give you back control, but touching the ground gives you control back immediately, allowing you to jump & activate the transition. The timer from the transition to give you control goes to zero, thus giving you back control, and allowing you to hit the transition again. You might regain control inside the transition hitbox, which makes you fall onto the platform you jumped up from - jumping again allows you to hit the transition a second time.

Effects
Duplicates a room.

All non-unique objects/enemies get duplicated (including grubs, background objects, hazards).

Second, there is a concept of main, active room. This is the "oldest" room in a dupe. When going through a transition while having duplicated rooms, the oldest room gets replaced by the new room.

For example, in the diagram there's three rooms, A, B, C. Transitions are coloured rectangles (so A-B is blue, B-C is red).

If you dupe A, you have "A+A". You go through (one of the) blue transition, transforming the "oldest A" into B. The other A becomes the oldest (active) room, B is the youngest one. Then you go through the transition from B to C. Even though it is a transition "in B", the room that gets replaced is the oldest one in the dupe : A, and hence you end up with a superposition of B and C (B being the oldest)

If going through overlapping transitions, which transition you actually go through is random.