There are a couple of reasons a device could receive what is perceived to be duplicate messages.
First, if you receive a WEA message and then delete it before the alert expiration, the device will have no record that the message had been received previously and will display the alert again when the network rebroadcasts the message (this is done at a regular interval). To keep this from occurring, when a WEA message is received do not delete the message until after the expiration time stamp included in the message. As long as the message stays in the inbox, the device will know the message has already been alerted and will not display the alert again (or provide notification).
Second, if the message originator sends mulitple messages that have the same text and happen to have overlapping target areas, the device will receive and display multiple alerts because they are distinct different messages with different message ids. Should this occur, feel free to provide feedback as we are working to help FEMA and the message originators create/modify processes and tactics to ensure full coverage of emergency information while keeping customer intrusion to a minimum.