To troubleshoot, always fully erase your drive, per example:Had the same problem. On correct firmware. MP 2009. Trying to install to internal SSD drive. Haven't resolved it so far. Attempted twice, installer left a mess behind in /MacOS Install (or similar). Check your root folder. If you have that and want to clear it you have to disable csrutil in recovery mode to use rm -rf, then of course re-enable it. Please post back if you give a solution. (Q: Did you make any copy of the Mojave installer elsewhere? I did and suspect a connection.)
Code:
diskutil list
diskutil unmountdisk diskXX
sudo gpt destroy diskXX
You have to change diskXX to the disk you want to fully erase. You need two drives to do this or if you have just one drive, you will need to do this from the USB installer. You can't do this with your booted drive.