If the initial save versus poison is made then I believe there is a follow up immunity to that type of poison. So you can't just keep hitting an enemy with the same poison type hoping they will eventually fail the save.
I'm not certain how long the immunity lasts.
@JandK it's in the top, the immunity is referred to as "inoculated", it lasts 2 turns and is poison type specific, you can be inoculated against multiple types of poisons simultaneously though.
It's down to the poison staying on your weapon for a specific time. Some are cumulative.
