Remove duplicate entries in the method index, fix attribute links 

I only noticed the duplicates because they were showing up
in Sequel's RDocs: http://sequel.rubyforge.org/rdoc/index.html.
I've confirmed that the cause is not duplicate files, so I'm not
sure why the duplicates are there.

The links for the attributes were incorrect, since hanna uses
attribute-* as the anchor for attributes.  Looks like both
respond to aref, so just use that.