My gut instinct tells me that 3 links on one and the same page to one and the same web document do not count for more than 1 link on that page.
That is, if a PR9 document references your site trice I think it would be utter stupidity on the side of Google to count that as 3 PR9 links, no?
Given that surrounding text is taken into account, I can imagine that, heavily depending on the linking document's content, the overall value of one well placed link is more effective, less dilutive, than 3 links.
Now if you say, but Ruud, I meant linking out to 3 other sites -- than the distribution of your page's PR is spread over 3 links instead of 1. A link from a PR9 page with 10 links is different from a link from a PR9 page with 100 links.
Furthermore, how links, and other elements, are valued and weighed, can be influenced by position in document. <h1> before the body of an article carries more weight than using <h1> in your footer.
Does this apply to plurals? i.e. would 'mortgage' and 'mortgages' be classed as duplicate anchor text in this scenario?
Through stemming I believe that both these examples can already mean the other...
I don't think there is such thing as duplicate anchor text. Sure, search engines can analyze and watch out for documents that seem to be linked to always with the same anchor text. By analyzing the linked document they can make an educated guess on whether or not someone is trying to game the system.
But true "duplicate anchor text", as in the meaning of a link, can only be achieved through identical surrounding text.
This <a href="http://www.example.com">fruit</a> cake recipe ...
He is a <a href="http://www.example.com">fruit</a> cake ...
This <a href="http://www.example.com">fruit</a> basket ...
The above have "duplicate anchor text" but by adding the surrounding text these links would automatically bubble up higher for the right context query.
I think that what you might have heard is how it is good to vary your anchor text and how repetitive anchor text for your links can be, but isn't always, damaging.
The variation comes not from one and the same link nor from one and the same document. It is one document linking with "fruit cake recipe", one with "cake recipe", another with "recipe", another with "cake". Think of it as tagging. The more accurately you tag a link on delicious, the more likely you are to find it back in the right context next time.