When creating or editing a page in a custom namespace on a wiki with VisualEditor enabled, the "Create source" and "Edit source" buttons appear to be duplicated. Their functionality is not affected -- the button on the left loads the VisualEditor normally, and the one on the right loads the wikitext editor normally -- only the display text is incorrect for the button on the left, which should simply read "Create" or "Edit", without the "source" part at the end.
I noticed this issue on https://sims.miraheze.org/. I created a "Fanon" and a "Game guide" namespace for the wiki, which is when I noticed the issue. I just checked and the issue seems to have "fixed" itself; the first "Edit" button for VisualEditor editing no longer appears on either namespace, leaving just the "Edit source" button.