double (v.)
late 13c., "make double," from Old French dobler, from Latin duplare, from duplus (see double (adj.)). Meaning "to work as, in addition to one's regular job" is c.1920, circus slang, from performers who also played in the band. Related: Doubled; doubling. To double up bodily is from 1814.
There's just one explanation: "double" in this context does NOT mean "double." The manuals mean what the rest of us mean by "lengthening."
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