Run into a situation where at some the internal links popup started including the iframe version plus the normal article.
If we set the media type to Internal Links it shows.
If we set the media type to "not set" then it shows as expected (minus the CSS stylings - which we want).
Doesn't seem to matter if it is 2.1.0 or 2.1.1, same issue.
I never noticed it but that isn't something we frequently review so not sure when this started happening.
My guess is some other extension interaction but I couldn't guess what. Maybe sh404sef?