Recently been using Rune of spellweaving (legendary). Rune page and description says "10% chance when casting a direct-damage spell or Turn Undead against monsters to cast the same spell a second time without extra MP or reagent cost. PvE only; does not trigger against players."
Works great with direct damage spells like sunburst, vamp touch, eruption, etc. Can see the blue text in chat when it activates and the animation of the spell. However, as far as I can tell it never goes off when casting Turn Undead.