Storyboard Internal Inconsistencies

An easy way to fix this problem is to open up the Storyboard in TextEdit, then go down to the resources section at the end and delete any duplicate entries for images you see. They're sorted alphabetically, so it's fairly simple to do.

Saved the file, loaded up XCode and no more error messages.

This problem occurred for me Xcode 10.1 but was resolved. The error message provided a "Show Details" button.

Clicking on Show Details showed my problem, a duplicate.

Right-clicked on the identified storyboard (Main.storyboard) to open as source code.

Searched for the duplicate declarations and removed one of the duplicates.

The error message no longer occurs including when the storyboard is reopened.