An already enchanted Tetra tool can still be enchanted vanilla style even when the effect is already on there.

When you're in creative mode it seems as if you can apply enchants to everything. As tetra enchantments aren't stored the same way as the vanilla ones they would probably stack as well. As it's only available in creative mode I'll keep it until we find something that actually breaks.

Makes sense. Serves me right not testing in survival.