Few days ago my RHEL 7.4 started to upgrade itself to 7.5. After that, the dual display setting is broken again. Last time I fixed it in RHEL 7.4 by replacing the ATI driver with old el6 version.
To fix that, just remove ~/.config/mointor.xml and reboot. It comes back.
There is another issue is “System Tray Icon is no longer available”. Some old applications provide system tray icons, and there is an easy way to fix it:
Go Tweak Tool > Tweaks > Extensions > Enable Top icons.
Reference: Bug 1548446 – System Tray Icon no longer available in gnome 3.26 in RHEL 7 out of the box