Multiple births
In biology, the term multiple births referred to multiple siblings born during the same pregnancy. 

The term twins referred to two siblings born from the same pregnancy. Three siblings born at one time were referred to as triplets, while four were referred to as quadruplets. These siblings could look alike or different and were often close in age, being born only moments apart. 

The species identified by Humans as Greek gods had twins. One example was Artemis, who had a twin brother Apollo. ()

When Beverly Crusher was eight years old, she had a crush on an older child named Stefan. As her infatuation with him grew, she began to create the imaginary family the two had together in her head, which consisted of them being married and having three children: twin boys, Andrew and , and then later, a little girl, . ()

Jean-Luc Picard compared meeting William T. Riker's transporter duplicate, Thomas Riker, to being "a little like meeting someone's twin." ()

Curzon Dax once knew what were described as "gorgeous Ruji twin sisters." ()

Benjamin Sisko once mistook Nidell's psychoprojective alter ego, Fenna, as a twin sister. () 

Twinned Miradorns were not just twin brothers, but together, were a single self: two halves of one being. If one twin died, the other twin would anguish in the loss of self. Ah-Kel and Ro-Kel were an example of a twinning. () 

Vilix'pran began his budding, in 2371, having his first set of twins. ()

When Miles and Keiko O'Brien were considering having their second child, Miles asked "What do you say we try for twins..." Keiko told him that "I don't think that's the way it works" and that "you'd better brush up on your biology." ()

The Delaney sisters who served aboard  were twins. () 

In the 20th century Earth soap opera viewed by Neelix and Kes, Neelix expressed a desire to see if Blaine's twin brother was the father of Jessica's baby. () 

Clones could often be misidentified as twins, triplets, or even quadruplets. () 

In Tom Paris' adaptation of The Doctor's holonovel Photons Be Free, in which Seven of Nine was originally envisioned as Three of Eight, was re-envisioned a group of triplets: One of Three, Two of Three, and Three of Three. ()

In the alternate reality,  had an encounter with a pair of Caitian twins in 2259.  also mentioned having delivered a set of Gorn octuplets once. ()

See also
Litter
Siamese twins
The Twin Mistresses of Evil
Twin moons
Twin planets
Twin stars
Xahea and the Xaheans

Background information 

Identical twins were prominently used in casting the episode . The use of these twins for each android "series" aided the photographic-effects budget for the episode. With imaginative use of twins and split screens, as many as six of one model were shown at once, while two of the same model required nothing but an additional costume. This ultimately gave the illusion of a planet of thousands of androids. (The Star Trek Compendium)

Twins were also often jointly cast in the role of a young child character in order to respect child labor laws. These included: 
 Jessica and Vanessa Bova (young "Alexandra", )
 The Tadeski twins (baby "Molly O'Brien", ) 
 Candace and Nicole Villwock (baby "Deanna Troi", )
 Matthew and Carolyn Corley (baby "Miral Paris", )
 Grace and Riley Norris (baby "Elizabeth", ).

In the case of the actors playing twin characters Azan and Rebi (Kurt and Cody Wetherill), twin stand-ins (Matthew and Nathan Kennedy) were also used for shots from behind and partial shots for this same reason.

Apocrypha 
In the novel Mission to Horatius, ancient Vulcans worshiped a god named Maripol, who caused earthquakes and floods. He could only be placated by sacrificing the hearts of twins.

In the Pocket TOS novel Savage Trade, it was revealed that Vulcan twins shared a platonic mental connection that prevented them from developing a mating bond with another Vulcan. They could therefore have a relationship with whomever they wished.

According to the eBook The Cleanup, 90% of the Miradorn population consisted of telepathically-linked twins.

According to the video game Star Trek: Starship Creator, Taurik and Vorik were twin brothers.

External links 
 
 
Groups
Sex and reproduction