Seeing your problem in an SEO viewpoint, it’s always best for a website not to have any duplicate content. So maybe try linking to the source of the listing on the IDX website.
Your rel="canonical" is in place and in the section where it needs to be.
The duplicate content maybe coming from what you are not doing, but what other similar sites are doing. How many other real-estate sites use the same identical keyword and description for the same listing as you? These similar listings on "other sites", could be the cause for the duplicate content issues on your site. I guess my question would be how many other sites have a house listed @ 20615 Wild Springs Dr, San Antonio, TX 78258 (MLS # 1034019) using the same address and description as you?
My understanding this is a common problem with IDX, not sure if this solves your problem, but may solve why you are having a duplicate content issue.