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The Collaborative International Dictionary of English v.0.48:

Diplex \Di"plex\, a. [Pref. di- + -plex, as in duplex.] (Teleg.) Pertaining to the sending of two messages in the same direction at the same time. Diplex and contraplex are the two varieties of duplex. [Webster 1913 Suppl.]
The Free On-line Dictionary of Computing (30 December 2018):

diplex (From telegraphy) Two simultaneous transmissions in one direction. Compare: duplex. (2000-03-30)