The effects of a duplicate MAC address can be subtle in some cases.

Switches distribute traffic to hosts based on "seen MAC" addresses. When you turn on your computer and it sends its first packet out on the network, your switch will log in its MAC table that "MAC address X came from port Y". Conversely then, in the future when it sees a unicast packet addressed to MAC address X, it knows to send it to port Y.

Since your VM is only on a single physical switch port, it's up to your hypervisor (VirtualBox) to sort out where to send the packets directed to that virtual MAC. In the case of a duplicate, it probably just sends it to both VMs and lets the network stack on each VM sort it out. (the networking stack would likely see that traffic was sent to its MAC address that did not belong to one of its own IP addresses, and silently drop the packet.) So you can imagine that this would cause a fair amount of extra work, for the OS to wake up and process each packet, whereas if you had unique MAC addresses the [virtual] hardware or driver could drop the packet intended for the other host, before sending it up the stack.

On a switched network (unlike your VM example), a duplicate MAC address would cause a switch to be confused about where to send traffic. Each packet a host with a duplicate MAC sends out would typically cause the switch to surmise that the host "moved" from one port on the switch to another. If both hosts were sending and receiving traffic at the same rate, you would expect each host to lose 50% of its return traffic.

ARP and IPv4 may not be too concerned about duplicate MAC addresses, so IPv4 networking may work properly. (though a robust stack, or a host with additional security/networking tools, may consider a duplicate MAC address as a red flag.) Also, if you are using DHCP, a DHCP server (absent a sufficiently unique client ID) could assign a duplicate IPv4 address, which could be problematic.

On the other hand, IPv6 bases automatically configured addresses on the MAC address. IPv6 also includes the concept of duplicate address detection, which means that a duplicate MAC address could cause the following effects (according to RFC 4862 section 5.4.5):

-  not send any IP packets from the interface,

-  silently drop any IP packets received on the interface, and

-  not forward any IP packets to the interface (when acting as a
   router or processing a packet with a Routing header).

The effects of a duplicate MAC address can be subtle in some cases.

Switches distribute traffic to hosts based on "seen MAC" addresses. When you turn on your computer and it sends its first packet out on the network, your switch will log in its MAC table that "MAC address X came from port Y". Conversely then, in the future when it sees a unicast packet addressed to MAC address X, it knows to send it to port Y.

Since your VM is only on a single physical switch port, it's up to your hypervisor (VirtualBox) to sort out where to send the packets directed to that virtual MAC. In the case of a duplicate, it probably just sends it to both VMs and lets the network stack on each VM sort it out. (the networking stack would likely see that traffic was sent to its MAC address that did not belong to one of its own IP addresses, and silently drop the packet.) So you can imagine that this would cause a fair amount of extra work, for the OS to wake up and process each packet, whereas if you had unique MAC addresses the [virtual] hardware or driver could drop the packet intended for the other host, before sending it up the stack.

On a switched network (unlike your VM example), a duplicate MAC address would cause a switch to be confused about where to send traffic. Each packet a host with a duplicate MAC sends out would typically cause the switch to surmise that the host "moved" from one port on the switch to another. If both hosts were sending and receiving traffic at the same rate, you would expect each host to lose 50% of its return traffic.

ARP and IPv4 may not be too concerned about duplicate MAC addresses, so IPv4 networking may work properly. (though a robust stack, or a host with additional security/networking tools, may consider a duplicate MAC address as a red flag.) Also, if you are using DHCP, a DHCP server (absent a sufficiently unique client ID) could assign a duplicate IPv4 address, which could be problematic.

On the other hand, IPv6 bases automatically configured addresses on the MAC address. IPv6 also includes the concept of duplicate address detection, which means that a duplicate MAC address could cause the following effects (according to RFC 4862 section 5.4.5):

-  not send any IP packets from the interface,

-  silently drop any IP packets received on the interface, and

-  not forward any IP packets to the interface (when acting as a
   router or processing a packet with a Routing header).

The effects of a duplicate MAC address can be subtle in some cases.

Switches distribute traffic to hosts based on "seen MAC" addresses. When you turn on your computer and it sends its first packet out on the network, your switch will log in its MAC table that "MAC address X came from port Y". Conversely then, in the future when it sees a unicast packet addressed to MAC address X, it knows to send it to port Y.

Since your VM is only on a single physical switch port, it's up to your hypervisor (VirtualBox) to sort out where to send the packets directed to that virtual MAC. In the case of a duplicate, it probably just sends it to both VMs and lets the network stack on each VM sort it out. (the networking stack would likely see that traffic was sent to its MAC address that did not belong to one of its own IP addresses, and silently drop the packet.) So you can imagine that this would cause a fair amount of extra work, for the OS to wake up and process each packet, whereas if you had unique MAC addresses the [virtual] hardware or driver could drop the packet intended for the other host, before sending it up the stack.

On a switched network (unlike your VM example), a duplicate MAC address would cause a switch to be confused about where to send traffic. Each packet a host with a duplicate MAC sends out would typically cause the switch to surmise that the host "moved" from one port on the switch to another. If both hosts were sending and receiving traffic at the same rate, you would expect each host to lose 50% of its return traffic.

ARP and IPv4 may not be too concerned about duplicate MAC addresses, so IPv4 networking may work properly. (though a robust stack, or a host with additional security/networking tools, may consider a duplicate MAC address as a red flag.)

On the other hand, IPv6 bases automatically configured addresses on the MAC address. IPv6 also includes the concept of duplicate address detection, which means that a duplicate MAC address could cause the following effects (according to RFC 4862 section 5.4.5):

-  not send any IP packets from the interface,

-  silently drop any IP packets received on the interface, and

-  not forward any IP packets to the interface (when acting as a
   router or processing a packet with a Routing header).

The effects of a duplicate MAC address can be subtle in some cases.

Switches distribute traffic to hosts based on "seen MAC" addresses. When you turn on your computer and it sends its first packet out on the network, your switch will log in its MAC table that "MAC address X came from port Y". Conversely then, in the future when it sees a unicast packet addressed to MAC address X, it knows to send it to port Y.

Since your VM is only on a single physical switch port, it's up to your hypervisor (VirtualBox) to sort out where to send the packets directed to that virtual MAC. In the case of a duplicate, it probably just sends it to both VMs and lets the network stack on each VM sort it out. (the networking stack would likely see that traffic was sent to its MAC address that did not belong to one of its own IP addresses, and silently drop the packet.) So you can imagine that this would cause a fair amount of extra work, for the OS to wake up and process each packet, whereas if you had unique MAC addresses the [virtual] hardware or driver could drop the packet intended for the other host, before sending it up the stack.

On a switched network (unlike your VM example), a duplicate MAC address would cause a switch to be confused about where to send traffic. Each packet a host with a duplicate MAC sends out would typically cause the switch to surmise that the host "moved" from one port on the switch to another. If both hosts were sending and receiving traffic at the same rate, you would expect each host to lose 50% of its return traffic.

ARP and IPv4 may not be too concerned about duplicate MAC addresses, so IPv4 networking may work properly. (though a robust stack, or a host with additional security/networking tools, may consider a duplicate MAC address as a red flag.) Also, if you are using DHCP, a DHCP server (absent a sufficiently unique client ID) could assign a duplicate IPv4 address, which could be problematic.

On the other hand, IPv6 bases automatically configured addresses on the MAC address. IPv6 also includes the concept of duplicate address detection, which means that a duplicate MAC address could cause the following effects (according to RFC 4862 section 5.4.5):

-  not send any IP packets from the interface,

-  silently drop any IP packets received on the interface, and

-  not forward any IP packets to the interface (when acting as a
   router or processing a packet with a Routing header).