I'm certainly not a PHP expert, and I'm not familiar with this
extension per se, but it would seem to me that detecting whether the
current version was also sighted would be as easy as comparing
revision ids. If the current ID and the currently sighted ID are
identical, don't show the link.
Maybe I'm over-simplifying, and sighted revisions are not marked
according to revision number, like I said I'm not familiar with this
code.
--Andrew Whitworth