Mac Pages, display glitch in format sidebar

Formally Quit Pages.

Click on the Desktop to focus on Finder, and then press shift+command+G to open the Go to Folder panel. Copy/paste the following to land in your local library's Caches folder.

~/Library/Caches

In icon view, look for an obvious file bearing the yellow Pages icon. Drag it to the Trash and empty the Trash. Close the Finder window. Holding the shift key, launch Pages. Has the double-image issue resolved?

If it did not go away, reboot into Safe Boot mode (by holding the shift key), and see if the double-image issue remains. Reboot normally, and try Pages again to see if the duplication persists. Status?

Better see your optician. 🤓 I cannot reproduce that with Pages v7.3 on macOS High Sierra 10.14.3 (18D109).

Was this problem present with Mojave prior to 10.14.3, and have you installed any third-party software prior to this problem occurring?

Good. The shift key was added insurance to prevent Pages from loading its prior application state, just in case it had crashed on you. Probably not needed in this case.