@Riot Question about Vel'Koz's Q - Does the refund stack?

I haven't found this explained neither have I found nonPBE discussion where I could ask. If this was explained already, then, of course, Riot response is not necessary, anyone can inform me. Does his Q Mana refund stack? With hitting unit and then splitting, though not so easy to do, it has potential of killing up to three units. Does this mean he can refund half the mana cost, make the spell free on mana up to making the spell free while additionally refunding half the mana cost (150%)? Or does it mean once at least one unit dies in any part of the ability, it refund only 50%?
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