Present participle of the archaic verb 'duplify'; the act of doubling or making twofold.
From 'duplify' with the present participle suffix '-ing'. This is an archaic form that's virtually never used in contemporary English.
Following 'duplify' into obscurity, 'duplifying' is essentially a ghost word—it appears in historical texts but modern speakers have no idea what it means because they use 'duplicating' instead!
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