Attached is a patch which does the following:

Modifies rcirc-browse-url to only complete to buffers that are before
point in the current buffer. To accomplish this, the structure of
rcirc-urls has changed from being just a list of strings to a list of
strings and buffer positions.

Modifies rcirc-markup-urls to conform to the new structure of
rcirc-urls and so that any URL that is already the latest recognized
URL is not added to the list of URLs. This prevents "consecutive
duplicate URLs" from being stored.