Same issue after upgrading from Ubuntu Gnone 17.04 to Ubuntu 17.10.
First I purged gnome-screensaver, but it didn't work.
Then I went to Setting -> Privacy -> Screen Lock -> Automatic Screen Lock and turned it off. Now the original Gnome lock screen went away and only one lock screen is left.
Same issue after upgrading from Ubuntu Gnone 17.04 to Ubuntu 17.10.
First I purged gnome-screensaver, but it didn't work.
Then I went to Setting -> Privacy -> Screen Lock -> Automatic Screen Lock and turned it off. Now the original Gnome lock screen went away and only one lock screen is left.