My personal experience is quite the opposite. Repeated exposure to poison ivy resulted in worse symptoms each time, leading to a scar from one particular welt that lasted years.

Something similar happened to my father (we had moved to a new house that had a large patch that kept coming back) and the year before he finally managed to get rid of it, his reaction was so bad he actually couldn't eat cashews for a long time, since they can have traces of the urishol.