Information on repeated data in the sensor history
One of the strengths of the Monnit protocol is that the sensors can both transmit and receive. After a sensor transmits its data it waits to hear an acknowledgement back from the gateway that the sensor data was received. If the sensor does not hear back from the gateway in a certain period of time it will retransmit the data. By default the sensor will resend up to three times if needed before it goes back to sleep.
The most likely cause of the duplicate data is that the sensor sent the data and the gateway received it, but when the gateway sent the acknowledgment back to the sensor, the packet was lost. The sensor resent the data to the gateway, then the second acknowledgement made it to the sensor and the sensor went back to sleep (Turned off its radio).
There are several reasons that packets can be lost. The most common are weak signal strength or another sensor (or other equipment in the 900Mhz range) transmitting at the same time as the wireless gateway.