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As said before, you can't duplicate items.
Now, about double spawnpoint opening in order to swarm opponent, I would do it differently.
In the given example I will use skeleton minions and skeleton sentries, but zombie crawlers and unstable zombies cost even less so they could be spawned the same way:
after 1st channeling starts with 20 mana
1) Death Ring + Libro Mortous (total cost of 14, since the ring give discount for libro)
after 2nd channeling 16 mana and 1 on libro
2) Skeleton Sentry + Meditation Amulet + meditation as full action (cost 10 mana plus 1 from libro, but regain 3 mana in meditation )
19 mana 1 on libro
3) Skeleton minion + graveyard + meditation (cost 17/1 regain 3)
15/1/1 mana
4) skeleton minion + skeleton sentry + Harmonize on graveyard + meditation (cost 13/1/1 regain 3)
And after 5th round channeling, you have 15 mana, 1 on libro and 2 on graveyard. Your total channeling (including spawnpoints) is 13 and you gain 3 mana from meditating, so with extra 2 discount you could get a total of 18 mana usage per turn.
I don't like harmonizing libro, since it would make it almost certainly dissolved.