"the last one" means the last thing in a series: I'm sick of peeling potatoes; thank goodness this is the last one. I've nobody else to see after you; you're the last one. "the last of it" is a colloquial expression referring to the termination of some activity, the abandonment of some idea, plan or course of action, or similar: That was a ridiculous idea; hopefully we've now heard the last of it.
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