Loopback interface on Windows: duplicate packets (inbound + outbound) captured

[Berkay Turan]

When capturing traffic on the Windows using PcapPlusPlus, each localhost packet appears twice: once on the send path (outbound) and once on the receive path (inbound).
Wireshark shows only one copy, but with PcapPlusPlus I see duplicates sometimes.

However, strangely, sometimes the same code shows only one copy.
So behavior is inconsistent: sometimes duplicates, sometimes single packet.

Steps to reproduce the behavior:
1. Install Npcap (latest version)
2. Run a simple program using PcapLiveDevice to capture on `\Device\NPF_Loopback`
3. Send UDP/TCP traffic to 127.0.0.1 between two local ports
4. Observe that `onPacketArrives` is triggered twice for each logical packet

Ideally, when capturing on loopback, PcapPlusPlus should behave like Wireshark:
- Show only one copy of the packet (either inbound or outbound)
- Or provide a flag/option to suppress duplicate packets

Additionally, from my debug logs I can confirm that the **same packet arrives twice**.  
Both copies have the same src/dst IP and ports, same length, and same payload,  
but their **timestamps are slightly different**.  
The timestamp value is taken from the function provided by the PcapPlusPlus library itself.

Environment:

- OS: Windows 11
- Npcap version: 1.79 (loopback adapter installed)
- PcapPlusPlus version: 25.05 (built with MinGW)
- Compiler: MinGW-w64 GCC 15.1.0

[PcapPlusPlus Support]

Hi Berkay,

I'm not sure how Wireshark deduplicates the packets. PcapPlusPlus is basically a wrapper around Npcap/libpcap and exposes all packets captured by these engines.

Maybe you know how Wireshark does it?

Thanks,

PcapPlusPlus maintainer

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