I was not able to reproduce packet duplication with enabling routing and even installing the VirtualBox. However, can you try to uncheck VirtualBox Bridge Networking Driver and see if packet duplication still takes place?
In the past I have observed packet duplication when working over my version of Ethernet Bridge based on NDIS IM driver. In order to get brdged to the real network NDIS IM driver has to place the real network interface into the promicsuous mode in order to be able to get packets destined to Ethernet address different from the NIC hardware address. So you have the cocktail of routing and bridging in the promiscuous mode on your system and adding one extra binding into the configuration (like LWF filter driver) in theory may cause packet duplication. So it makes sense to remove Virtual Box Network bridging from the configuration to see if there is any difference.