It turns out the lightbox with class names starting with MFP is the “Magnific Popup” which is loaded by Divi. This is supposed to be able to be disabled by going to Divi, Theme Options, and disabling the “Divi Gallery”. Unfortunately, doing so didn’t stop the duplicate lightbox in my case.
Sundari Webdesign has a guide to disabling the Divi lightbox using a hack, which they say is now obsolete due to the configuration option above.
In the end the Divi configuration option didn’t work for me, and I wasn’t ready to try the hack to disable the Divi lightbox, so I just disabled the SLB lightbox instead. It’s unfortunate because the SLB lightbox has more features that I wanted. Hopefully someday the Divi lightbox will be more easily replaceable by alternative lightboxes…