First, try to update the ir-pcsx-rearmed core and check again . instructions:https://retropie.org.uk/docs/Updating-RetroPie/.
If after the update the problem is still there, follow this steps:
To disable the multitap:
Open the RetroArch menu while the game is running and from the options, enable the Show other input settings.
After that, restart the game and open again the RetroArch menu and from the options set to off the Mutitap1 and Multitap2 and you are ready.