I'm trying to upload both images here via batch upload: https://minacream.tumblr.com/post/189021650340/
The first is a picture of Helena and Cleveland from Azur Lane, ballroom dancing in appropriate outfits; the other is a variant: same characters, same post, but wearing lingerie.

The problem is, when I try to upload the second, it identifies it as a duplicate of the first (post #3809755). I don't understand.

There may be some bug in the bookmarklet where it grabs the first image in that set instead of the second, I think there have been some issues before with other sites. Did it show you the right image on the upload screen and then give you the duplicate error when you submitted? or the wrong image on the upload screen?

You could try saving the image to your HD and uploading from there, if you can figure out tumblr's weird full size / sample URLs (they've changed it after i knew how to do it)

I uploaded the second image by itself without using batch upload; it worked.