Welllllll shit. Figured it out I think. I have an add-on that lets the site automatically insert attached images without having to tap thumbnail or full.
By default, if you attach an image and click reply, it will load as a thumbnail. So tapa will double post an image b/c the add-on is skipping the decision step, if that makes sense.
I like having the add-on b/c it eliminates the need to do an extra step. I'll have to holler at the dev.