Upon further examination, I see that the additional text is also a mapped Content Control with a label "Subtitle." That label is not one used by the standard Document Property Content Controls. Indeed, the Subject Document Property Content Control (DPCC) was renamed "Subtitle" and repurposed as described in my article.

Here is what I see if I insert the Subject DPCC in another location in the document. The text that was inserted in the Subtitle content control is automatically repeated.