How do I stop random images from repeating?

I found some code that creates four graphics “cards” coming up randomly on the page [url]http://opdr.net/relationship_reading.html[/url]

but the problem is they are repeating themselves.

How can I change the script on that page so the 2nd card that comes up is different than the 1st card; the 3rd is different than the 1st and 2nd; and the 4th is different than the 1st, 2nd and 3rd?

Any help would be greatly appreciated.

Thanks,
Chris

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