The Gemini Effect was the name give to the project of temporal research conducted by Doctor Paul Manheim in 2364. The effect itself creates a temporal disturbance in the space-time continuum that allows two same ships to exist simultaneously in different locations. The effect was temporary and the duplicate ship would eventually disappear but this did allow Starfleet to bolster in numbers in short engagements against the enemy.

Nebula-class vessels were capable of accomplishing the effect, which they used to create duplicate vessels to bolster a Federation fleet. (TNG video game: Armada)