New Collaborator Cannot Add Layout Skin in Responsive HTML5 Settings to Team's Project

I don't think I've ever seen a case where the dropdown list doesn't show all the  Responsive screen layouts that have been added to the project already. For example, in my test project I have a bunch 🙂 

Time for a few random suggestions. As always, make sure you have a back up, just in case.

Perhaps try adding one of the other screen layouts, to see if that can prompt RH to display all the screen layouts in the dropdown list.

Try closing the project, deleting the cpd file using File Explorer, then re-open the project. Check if the screen layout now displays. If the project is very large, it can take a bit of time to finish regenerating the cpd file so don't panic if you have to wait.

Confirm you don't have the .cpd, .ldb, or .pss files checked in to source control - these files contain information specific to a pc/user and have caused things like topic additions and deletions not to appear for each user.

Do they just select it from the dropdown list in the Output Settings dialog box? Clicking the Gallery button will try to add a new screen layout to the project giving the error you see if the screen layout already exists, whereas the dropdown list will apply an existing screen layout from the project.

I'm not clear what you see. Do you see screen layout in the Screen Layouts folder?

Does the layout appear in the Output Setup pod under Screen Layouts?  The drop down list in the settings dialog box should show all layouts that appear in that location.

If it doesn't appear in that location, right-click on the Screen Layouts folder and select Explore. Can you see a folder with the correct name in Windows Explorer? Let me know what you see, before I make any further suggestions.