I have never seen this before. I searched for widget supplies and the number 1 site is also listed #18. The number 1 listing is his homepage ie mydomain.com and the second listing is an internal category page domain.com/store/widgetsupply.htm.
My experience has always been that the second listing would com as an indented sublisting of the first listing.
I've seen it. It tends to be when one of the pages is "freshed" and the other is coming from the main index. I've even seen the same pages come up differently such as the "freshed" version as result #2 and the regular crawl version #15.