The link cache is constructed without FOR UPDATE, so collisions in these insert state...

The link cache is constructed without FOR UPDATE, so collisions in these insert statements appear to be possible, and judging by the DB error log and reports of link table corruption, quite common. Ignoring errors prevents the wholesale failure of a link table update, although it's possible links which should have been removed may remain. Hence this is a temporary solution, a better solution may be to switch LinkCache to FOR UPDATE mode