In a deceitful, dishonest, or double-dealing manner; with duplicity.
From 'duplicitous' (adjective) with the adverb-forming suffix '-ly'. Traces back through 'duplicity' to Latin 'duplicitas'.
Someone acting 'duplicitously' is like an actor playing two roles simultaneously—pretending to be trustworthy while secretly pursuing opposite goals. It's calculated, conscious deception, not accidental dishonesty.
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