How to merge multiple objects with addition?

I have a single color  image step and repeated 12 times.  I'd like the images to add to each other where they overlap.

I've tried 50% transparency on the objects before doing the step and repeat.  This prevents the images at the top of the layer from totally obstructing those under it, but still the more images overlap a single area on the canvas, the dimmer it appears.

Is there a easy way to get them to add where they overlap, versus on top of each other with transparency?