Scheduled posts problem

I’m wondering if anyone can help.

I have a user of my plugin and when he posts a post directly everything is fine but if he schedules the post then when the post gets published my code (which is hooked in using add_action('publish_post', 'wordbooker_publish');) fires twice.

I can’t reproduce in on my set up and no-one else seems to be complaining about it which would suggest its either a “rogue” plugin or a configuration issue in his system.

Has anyone else ever experienced anything like this?

At the moment I’m looking at having to code in a check to make sure my code isn’t already running for that post.