Repeated short term exposure to heat (without heat stroke) causes many of the same adaptations as exercise training including plasma volume expansion, improved cardiac output, decrease sweat sodium concentration, and increased thermoregulation capabilities. This makes a person less likely to suffer heatstroke.

I would imagine there could be permanent damage related to severe heat stroke, but I would be surprised if it made people more likely to suffer subsequent heat stroke (as one commenter liked to an article about). Anecdotally I know many people who’ve been hospitalized with heat stroke but have no issues in the heat subsequent to the initial incident