Are there diminishing returns for having multiple backlinks from one site? For example if there is a relatively high PR site, lets say 7, and you get multiple backlinks from different pages. Would you get more PR juice than just 1 link?
Multiple links from the same site still count as multiple links. Still, I have heard that they are slightly devalued. Nothing major, though.
In regards to Pagerank as your question seems to revolve around, yes. Each web page has it's own Pagerank value, so if different web pages on a site link to yours they will each pass on their respectivie Pagerank value; with regards to other outbound and internal links ofcourse.