Says you can turn an object into a creature.
Of the object is a Clone duplicate and the creature is a 20th level Wizard, does the new creature gain the spell abilities?
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For starters, the object you use doesn't really matter. You'll get the same results whether you polymorph an inactive clone or a stone. Since the CR limit for an object is 9, and a 20th level character is above CR 9 (based on the Archmage NPC in the MM), it won't work.
Secondly, it's unclear whether you can duplicate a particular creature using True Polymorph, so that would be the DM's call. In other words, while you could transform the 20th level fighter into an Adult Red Dragon, it's less clear whether you can transform him in Balphorzeon, a particular adult red dragon that you met the other day.
Personally, I would recommend against allowing polymorph to duplicate an individual, since then you could arguably just polymorph a party member into the BBEG (or one of his trusted advisors if the BBEG's CR is too high) and have the party member tell you everything they know (the polymorphed individual retains their own mind, therefore they ought to be cooperative).
Additionally, you enter into an abusive situation where a wizard can true poly himself into himself with full spell slots, and wait for the poly to be permanent. Then, when he's running low on spells, he casts dispel magic on himself to recover all of his spell slots. If he runs through most of his spell slots again, he can just poly into a version of himself with full spell slots. That's almost certainly not RAI.
If you can't duplicate an individual, then duplicating knowledge possessed by an individual is also arguably not possible. Therefore, while you can poly someone into a human, you can't poly them into a human wizard or a human fighter.
Like I said, it's up to your DM, but I wouldn't allow it IMC. Even when considering it's only available from levels 17+, it's too easy to abuse.