It is common that a single packet capturing device (such as a passive prove, or any other passive tool analyzing the network performances) may be fed by copies of the same data in the same network.
Usually this happens when customers use SPAN (also referred to as Port Mirror) technology to collect multiple VLANS or multiplied ports on a switch/router to the same packet capturing device. In this case, the packet capturing tool may see the exact same IP packet multiple times even though there was no poor network performances.
One simple example is in the figure below: PC-A in VLAN 1 sends IP packets to PC-C in VLAN 2. These packets are routed at the multilayer switch. At the mirror port of the switch, we obtain both the ingress and egress copies from every packet belonging to this stream, which constitute duplicate packets. Clearly, monitoring only the ingress traffic to this port avoids data duplication, but in this case, a packet coming from PC-B to PC- A or PC-C would not be captured.