The quality or state of being duplex or double; the condition of having two-fold nature or functioning in two ways simultaneously.
From duplex plus the suffix -ity (forming abstract nouns). This creates a parallel to complexity, simplicity, and other -ity nouns describing qualities.
The suffix -ity is one of English's most productive—you can add it to almost any adjective to create an abstract noun describing that quality, which is why duplexity works.
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