You can improve on plague, or any other malediction spell like it (blindness, poison, curse, etc.), even if the target already is affected by it.

For the same reason, I like to just master wands and excision using a wand with a self-buff the class already has that can be gathered while you spam it. Doesn't matter that you already have the buff on, you'll still improve in it from wanding it on yourself.

The aura/barrier/shield system gives you wands that reduce incoming damage, your guild master will hint where to gather them when you discuss with them, when you're able to get them. Sleek amber comes at 30, gives you aura. Sleek sienna gives you shield, comes at 32. Sleek black gives you barrier, comes at 35.