Using WP for this was relatively straightforward except for several hours wasted in getting the indieauth plugin to work (a missing PHP dependency!!!), which I would never have solved without help from #indieweb-wordpress on IRC. I will ping you when I finish my notes on this.
As such this is a bit of a twist, beacuse the goal was to indiewebify my hugo site and to continue using the excellent ox-hugo! Now, I will probably use ox-hugo to power a 2nd static site as a brain-dump and experiment with indieweb.
Per Aaron – the repeated POST triggers is a default behavior. The webmentions.app script that I was using actually had to be called every time an article was published. Once. With that approach – I would probably have to create some form of a database, and/or add a post git / hugo hook to call the script.