For a while now whenever I use slackpkg it asks to upgrade:

gdk-pixbuf-0.22.0-i486-1.tgz
libgtkhtml-2.6.3-i486-1.tgz
slackpkg-1.5.2-noarch-2.tgz

which are the same versions already installed. If I say yes at prompt and run slackpkg again, it will present me with the same choice.

I've tried removing with pkgtool and installing afresh, tried a few different mirrors, looked around in /etc/slackpkg for clues but got nowhere.

Any ideas?

Btw - slackpkg works fine apart from this, and seems to upgrade everything else normally, so this is more of an irritation then a problem

Hard to say, slackpkg works ok for me. I'd suggest to update repo info files with slackpkg update. If it won't help I'd suggest to do a reinstall of objectionable packages.
Let us know if you've succeeded.

Yeah i usually do an update first and I tried removing gdk-pixbuf with pkgtool and then installing using slackpkg, but then I did a slackpkg upgrade-all and the problem persists

Ok think I've solved it. I had 2 different versions of slackpkg installed. Removed both with pkgtool and downloaded and installed from slackware site.

Then I had a closer look at the other pkgs with pkgtool and discovered I had also 2 different gdk-pixbufs and also an extra libgtkhtml (a gnome version). After removal of these everythings back to normality.

I'm guessing that updating slackpkg itself with slackpkg may be a problem and better to do that manually?