Since running headers never automatically add page numbers, it must be something on your master pages; there must be a frame containing both your running header placeholder (aligned left) and a page number code (aligned right).
You also cannot add a "Current Page Number" code to the "Text Before" or "Text After" fields, although on reflection it would have been an interesting option.
Actually, no. There was no separate frame there - just a single frame that added a page number. I finessed the issue by changing all of my "Head 1" headings to "Heading 1" and then changing the running header to link to Heading 1. The page numbers disappeared. Now my bookmark entries are whack - my second Heading 1 entry shows as a subsect of the first Heading 1 entry - but I'm on a deadline and will worry about that later.