In a dual or double manner; in two ways simultaneously; archaic form meaning 'in duplicate'.
From 'dupli-' (double) with the adverb suffix '-ly'. This is an extremely rare and archaic term, virtually unused in modern English.
If English had kept 'duply,' we might say 'I'll address this duply' meaning 'I'll handle it two ways at once.' Instead, languages naturally trim rare words, keeping only the most useful ones!
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