Including a featured image adds “-2” to post URL

This is a weird one and it took me forever to narrow down the behavior.

I’m having a problem with 4.7.3 whereas the URL for a post will have a “-2” appended to the end (i.e. http://www.wordpress.com/article becomes http://www.wordpress.com/article-2 but only if the post name and the featured image name are the same.

The “-2” cannot be removed. If you change the URL of the post to get rid of it, it just magically reappears.

However, permanently removing the offending image and re-uploading it with a different name (and re-pointing the post at the new image) solves the problem. To make it even more interesting, doing so sometimes makes the “-2” go away automatically (i.e. I didn’t have to re-edit the post to fix it).

Yes, I said sometimes. Sometimes the “-2” goes away after the image file name is changed and sometimes I have to edit it manually.

If the image name is different then the issue does not occur at all. It doesn’t matter if the image is a jpg, gif, png, etc as the file extension seems to have no bearing.

Via a process of elimination I was able to rule out any plugins I have installed (turned them off one at a time and tested) as well as my theme (changed theme and the problem still occurred).

I also turned off/cleared caching and the like (actually, I tried that first).

Anyone else seeing this? Any suggestions?