There's a difference between getting resistance to an element and getting the element,
as for simple armor enchants, it will be overridden, just as said, by the card.
The last slot that enchants your armor will be the element with it it's currently enchantent.
However you can get elemental resistance as much as you want, it will stack most likely.
If you're enchanted with a specific element, you just get a little resistance (if not congfigured otherwise, but this is default) to this element, not full resistance