Are you using SPAN to capture them? If you have SPAN configured on several ports on your LAN switch, and the packets traverse more than one port (eg, in one port, out another) SPAN may be copying the packet on ingress and also on egress.
Either reconfigure your LAN switch (eg, capture on only one port), or modify your application to deal with the duplicates. If you want a solution that works in a variety of environments, you really need to deal with the duplicates.
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