Even I had re same issue but I didn't have two guest user accounts but I know how to remove them go to System presences> Users&Groups then click on login options then click on network join then open directory utility then unlock the lock then click on directory editor then search guest and delete one of the guests.
To remove password from guest open terminal located in Applications>Utilities then type
sudo passwd guest it will prompt new password then type 'f' then again type
passwd Guest it will prompt 'old password' type 'f' as the password then it will prompt 'new password' don't type anything then again it will ask for 'Retype Password' again don't type anything. If you have any other problem comment below.