From The RadioReference Wiki

Duplex channel radio systems describe the use of two frequency channels, usually in the same band spectrum, which allows simultaneous communication between two stations. This is used to alleviate some of the problems encountered with simplex radio communication.

In the typical duplex type of system, the dispatch or main base station listens on one frequency (say Channel "A") and transmits on another (say Channel "B"). All of the mobiles (or local base stations) transmit on the same frequency that the dispatch is listening on ("A") and listen on the dispatch transmit frequency ("B"). This has the advantage of allowing, with the proper equipment, both the dispatch and a mobile station to talk and listen at the same time. In practice, this is impractical from the mobile’s standpoint; however, most dispatch stations in such systems are always listening even when they are transmitting. The main disadvantage of such a system is that even mobile stations that are close to each other do not normally hear the other mobiles because they listen on a different frequency than mobiles are transmitting on.

This type of operation is still in use for agencies that cover a wide area such as state police. Usually car-to-car communication is made between mobiles on the dispatch transmit frequency.

In the RR Database, duplex operation is shown as a two separate frequency entries (one with "B" in the "Type" column and the other with "M" in the "Type" column) and the "Description" indicating the paired relationship. Note that this is different from a repeater operation which is designated by the "RM" in the "Type" column and a single entry in the database. The way in which non-repeated duplex frequency pairs and repeater pairs are entered in the RR database is described in the Database Administrator Handbook.