Duplicity means deliberate deceitfulness, especially telling different people different things, while duplication simply means making a copy or repeating something. Despite sharing the root duplic-, one is about dishonesty and the other about repetition.
The core difference
Duplicity is deceit that plays two sides against each other, while duplication is the neutral act of copying or repeating something.
- duplicity — deliberate deceitfulness, especially by saying different things to different people: The negotiations collapsed once his duplicity was exposed.
- duplication — the act of duplicating, or a copy of something: The report flagged unnecessary duplication of effort between the two teams.
How to tell them apart
Both words trace back to Latin duplicare, "to double," but they took different paths from there. Duplicity kept the idea of a person doubling their face, showing one side to one audience and a contradictory side to another. Duplication stayed closer to the plain sense of doubling or copying something, with no suggestion of dishonesty.
A simple test: if you could replace the word with deceit or double-dealing, you want duplicity; if you could replace it with copying or repetition, you want duplication. The mix-up tends to appear in business writing, where a document might mean to flag repeated work (duplication) but accidentally imply dishonesty (duplicity).
