My home page exists at both .com and .co.uk. Both URLs point to the same page. However .com has a PR of 5, whilst .co.uk used to be 4 but now has zero PR. .co.uk has many many links to it, all relevant to the site's content, and a lot of them with good PR, whilst .co. has only a handful of links. It seems the more links, the lower PR.

Can anyone suggest a reason?

Duplicate content?

maybe it's some kind of Google bug; it happened to one of my sites (a drop to PR0), but that site had just several incoming links; I would wait for the next PR update to be sure;

When you say "point to the same page," do you mean that the .co.uk domain redirects to the other? If so, it's probably feeding its PR directly to the .com domain. Google traces the paths of redirections and can tell where a visitor is going to end up when a link is clicked. This same path-taking technology allows Google to supply sites with PR through "exit links" (for example, http://www.mydomain.com/exit.php?go=....newdomain.com).