Is this one of those "if you had a friend....(fill in) what would you do" questions?(nl);-)

The underlying question is a basic datacom one, related to error handling and error recovery. The first part of it is whether MMSI number is the used as the device "address" (like an IP number), or whether there is a hardware-specific address (like a MAC address on an IP network) that will be unique regardless. The next part is to learn how conflicts are resolved within that protocol. If the MMSI is used like an IP address you will get conflicts and one or both devices will not respond properly or may step on each other, so the result may or may not be predictable depending on the specifics. How the systems are programmed handle error recovery or MMSI duplication is key.