Multiple births

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

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

Humans were a species that 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 Alexander, and then later, a little girl, Jennifer. 

William T. Riker likened meeting his transporter duplicate, Thomas Riker, as 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 USS Voyager were twins. 

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

Clones can often be misidentified as twins, triplets, or even quadruplets. 

In Tom Paris' adaption of The Doctor's holonovel Photons Be Free, where Seven of Nine was originally envisioned as Three of Eight, became re-envisioned a group of triplets, including one named Two of Three. 

In the alternate reality, had an encounter with a pair of Caitian twins in 2259.

Background
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)