A sensitive species whose members involuntarily split into identical copies whenever they feel embarrassed, making any diplomatic contact with them a delicate exercise in tact.
Dooplers are short humanoids with a singular reflex: strong embarrassment or anxiety causes one to bud off a full-sized duplicate, which is every bit as easily mortified as the original. A single awkward moment can start a chain reaction.
They appear in the Lower Decks episode An Embarrassment of Dooplers. The Cerritos has spent days carrying a Doopler emissary to Starbase 25, with the crew tiptoeing around his feelings. At the last minute a small slip sets him off, and the ship fills with hundreds of fretful copies.
Captain Freeman finds by accident that anger works in reverse: when she finally shouts at them, the furious Dooplers merge back together. She then releases the emissary into a command party at the starbase from which she has been snubbed.