@veganpete said in Retropie launching ROMS from SD card, instead of USB.:

My question is:
1- How do I delete the duplicate ROMS from the SD card?

a number of ways. the docs are typically for adding roms, but instead of adding you can use these methods to access the files, and then delete:
https://retropie.org.uk/docs/Transferring-Roms/#sftp
https://retropie.org.uk/docs/Transferring-Roms/#samba-shares
https://retropie.org.uk/docs/Transferring-Roms/#manually-copy-files-from-usb-stick

2- How do I point Retropie to launch ROMS from the USB stick?

https://retropie.org.uk/docs/Running-ROMs-from-a-USB-drive/

3- How do I prevent Retropie from trying to copy the ROMS back to th SD card again?

doing https://retropie.org.uk/docs/Running-ROMs-from-a-USB-drive/ will stop that happening.