A rare or archaic plural form, or a group of two things in some technical contexts.
From Latin dupla, the feminine form of duplus (double). This form appears mainly in very old texts or in specialized terminology where Latin plurals are preserved.
Some English words keep their Latin plurals (like 'data' or 'alumni') instead of adding -s, which is why you occasionally see archaic forms like dupla in very old or technical writing.
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