There’s a serious dearth of FeedWordPress filter plugins out there. I aim to rectify the situation. FeedWordPress Collapse Filter will collapse multiple posts being imported from a single feed into a single post. I use it here on Plutor.org to keep Flickr photos (which I frequently upload by the handfuls) from overwhelming everything else. Each syndicated feed can be separately configured to collapse (or not) with a different time threshold.