A WebmasterWorld Thread reports that Google has been crawling double slashed URLs. For example, instead of crawling www.mysite/products.htm it would crawl www.mysite//products.htm. Apparently, this has been an "issue" since the summer. An other member reported this back in late November, as well. Plus there is Google Group Discussion from late October on the topic.
Update: Martin sent me an example of Google indexing double slashes. If you do a search on the "?id=" at Google, the bottom result shows you;
301ing those URLs to the standard slash, may help.
Forum discussion at WebmasterWorld.