A duplex adapter or duplex adapter part for a duplex plug of an optical plug-in connection includes two pairs of plug openings which are aligned to one another and into which in each case a duplex plug may be introduced from oppositely lying sides. It is formed from a housing and at least one frame which is fastened or fastenable on the housing and which projects proximally from the housing. The frame surrounds the two plug openings of an adapter side and comprises a middle portion which extends between the plug openings. A safety device for the plug side and which matches this, includes a duplex plug or a clip for creating a duplex plug from two simplex plugs, as well as an actuator for simultaneously activating unlocking pawls of both plugs.