I am using the Redirection plug-in, and I see that there is an option for "pass-through" rather than 301 redirect. I am interested in using this because my URLs will have a more consistent appearance. However, I'm not sure if it is essentially a 301 with masking or something else -- and I am not sure what effect it has on search engines. Do search engines crawl the source URL, the target URL or both? Are the source and target considered duplicate content when using pass-through? Thanks.