Resistance in Dungeons and Dragons does not stack. As with other spell mechanics, the highest effective level of spell takes priority for its duration and other, weaker spells don’t go into effect (however their duration timers still run.) However, there are times in which a spell might not explicitly use the word Resistance, instead saying the character “takes half damage from x source”. In that case, resistance would be able to stack, as the “half damage” isn’t under the resistance umbrella and is instead a stand-alone effect.