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I've already made the decision to stick to "wouldn't have been necessary," but I wanted to double check. For a little more context, the full sentence is as follows:
She filled the printer with paper, but if [this] had happened then it wouldn't have been necessary.
"Wouldn't have been necessary" is quite correct and a normal-sounding everyday phrase.
"Would have been unnecessary" sounds a little more formal and forceful, perhaps even less polite in some context.
