Comment by tsimionescu

You're using somewhat sloppy terminology that will confuse things. An IP packet can't be addressed both to 12.13.14.15 AND to 192.168.0.7.

The realistic attack here is that your ISP sends a packet with destination address 192.168.0.7 to the MAC of your router (the MAC that corresponds to 12.13.14.15). This is a realistic attack scenario if the device that your router connects directly to gets compromised (either by an attacker or by the ISP itself).

Getting a public route that would take packets destined for 192.168.0.7 to reach your router over the Internet is far more unlikely.