Web Personalisation; 1st page visited only

I'm sure I am missing something obvious here.

We want a web personalisation campaign to fire on the first page a user visits; but not on every page in that session. The segment is set to "On Entry" and the campaign to Any Page on our domain.

I recall, but am unable to locate, that web personalisation campaigns only fore once per session.

I was expecting that regardless of which page I enter the site I would see the campaign; but not on subsequent pages.  What we are seeing is that the campaign fires on every page visited.

Am I missing something or is it not possible to throttle the campaign to only appear once in a session.