Yea, I got that same message last night when I tried to flash JPM with 512.PT over my already-installed JPM (reasons for doing this are irrelevant).
What you gotta do is;
1 - Download JM8, JPM, Odin 1.3, 512.PIT and 803.PIT
2 - Flash JM8 with 512.PIT - select Re-partition option
3 - Once it's done and booted all the way to TW, go to recovery and wipe cache and factory reset
4 - Flash JPM with 803.PIT - make sure Re-partition option is NOT selected
5 - Once it's done and booted all the way to TW, go to recovery and wipe cache and factory reset
That worked for me
You can find all these files, flashing instructions, and booting to recovery instructions all over this forum.
Good luck, champ.