A pair of two closely related or connected things; sometimes used in physics or chemistry to describe paired particles or substances.
From duple plus the suffix -et (forming diminutive or small-scale nouns). This is a relatively modern formation modeled on doublet and triplet.
In physics, scientists use 'doublet,' 'triplet,' and 'duplet' to describe particles that come in groups—it's a way of borrowing familiar number language for the quantum world.
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