If you see duplicate consecutive sensor messages on one scheduled Heartbeat
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. You will generally want to examine the environment near the sensor for sources of interference. Removing the sensor from its environment and testing away from the current installation may be useful in determining if there is a source of interference near the sensor.
Start up test
It is important to distinguish two or more consecutive sensor messages from the start up test a sensor conducts when it is first powered up or joins a gateway. When a sensor first connects with a gateway, it sends consecutive messages as a handshake 30 seconds apart. Some sensors send 4 messages, and the most recent sensors send 10 messages 30 seconds apart.