I've noted that some targets don't have an MMSI listed -- I know that is not supposed to be so; however, i) it may be that some Type B units don't actually send-out or display properly for who knows what reason; or, ii) it may be that the boats haven't listed [by mistake or because they don't have a digital VHF].

In any event, I don't think it's an IP like issue.

In answer to the question... I think you'd see some goggledy-gook. If you had a 600' long cargo ship or tanker with two units, one transmitting located in the bow and one in the stern, then you'd have different reports of the "two" points. [The difference inrate of turns reported would be interesting at the start of the turns :^)

Or, if the units were close to one another, you'd have two reports on top of one another.

I think that the USCG should be bashing you about a bit if you get caught doing that. You're FCC license and USCG regs are aimed at providing correct and useful information -- you wouldn't be doing that. (It's similar to people with an open mic that is causing consternation, in my humble opinion)