You could try reinstalling IE6 and Sp1, then let it do an update.

I've just checked and I'm on IE6 SP2, so downloading SP2 will make this update redundant anyway.

The way it looks for updates is by checking file version numbers.

Sometimes, the files get updated by something else, and it sees the wrong version number so assumes the update hasn't been done, so does it again.

Failing that, do an Update but instead of letting it do the express install, select the advance one, and take that update out, tell it not to download it again.