"Main" network site will publish revisions of subdomain blog posts as new posts

I’ve got a WordPress network site at: https://phantompower.org/

For what it’s worth, this network began a couple years ago as a WPMU installation and was eventually upgraded to the 3.2.1 branch. Everything was working fine. But today, I noticed that some posts from the subdomain blogs were showing up multiple times in the main network site archive.

I deleted a bunch of these to clear things up. But to show you an example, I’ve created a test post:
https://wow.phantompower.org/2011/07/19/a-test-post-for-the-support-forum/

The first version of this post simply contains the text, “this is just a test post for the WordPress forum.” And as you can see on the main site at https://phantompower.org/ this post is there.

Now I went back to the original post linked above at the subdomain blog and added, “now here’s a second line of text” which shows up normally on the subdomain blog. But on the main site, there’s now a duplicate post that contains this revision below the original test post. This is happening anytime a post gets revised on any of my subdomain sites. If a post is saved on a subdomain blog and is then revised and saved again, a duplicate post will appear on the main network site that contains the revision. I went through the archives of the main site and found that this started happening in June. I’m guessing after I did the 3.2 upgrade.

Any ideas on how to fix this? It didn’t start happening until last month.