Are you saying they are both in uploads/auto_thumbs? Or is one in there and the other in uploads/catalogues?

Both in the auto-thumbs directory.

Thanks for the report.

I looked into it, and you're right, but it's for a reason.

The 168_168_* one represents a thumbnailed image
The other one represented a scaled-down image.

This is a very subtle thing. The thumbnailed image may be bigger, if the source image was smaller than the thumbnail size. This is because thumbnails have to be uniform. The scaled-down image is more appropriate for normal rendering of a field where you just want a maximum size, but never to get blown up to a minimum size.

Composr allows you to reference either in your templates.

You may ask why not just generate the thumbnail one on demand, as it's not used by default. I asked myself the same thing. It's because Composr is pulling in the thumbnailing templating too, which needs the image to be pre-generated.

I would have seen a difference if my image was smaller than the thumbnail created, so I do understand the reason for 2 images now you've explained it. I just saw the exact same dimensions in both and thought it was a duplication bug.

Why is it pulling in thumbnailing templating if it's not using the pre-generated thumbnail though? Isn't this something that should happen if you are referencing a thumbnail rather than the default scaled-down image? Maybe not, but it seems like a valid question in my mind (which doesn't comprehend how everything works in Composr like yours does).

Extra parameters are passed into catalogue templates in case you want a non-default layout and the default way fields are rendered are not sufficient to build that. Thumbnails would be a common thing people would want to include in their custom layout.