Full duplex refers to the transmission of data in two directions simultaneously. For example, a telephone is a full­duplex devicebecause both parties can talk at once. In contrast, a walkie­talkie is ahalf­duplex device because only one party can transmit at a time. Most modems have a switch that lets you choose between full­duplex and half­duplex modes. The choice depends on whichcommunications program you are running.
In full­duplex mode, data you transmit does not appear on yourscreen until it has been received and sent back by the other party. This enables you to validate that the data has been accurately transmitted. If your display screen shows two of each character, it probably means that your modem is set to half­duplex mode when it should be in full­duplex mode.   _{Your Comment}