Choose the right phrase for this awkward birthday party moment.

Oh no, you ______ that giant inflatable dinosaur for Leo! I already bought him the exact same one yesterday, and there is absolutely no room in his apartment for two of them.

The correct answer is needn't have bought.

The phrase needn't have + past participle is perfect here because the friend already performed the action (they bought the dinosaur), but the speaker is pointing out that it was unnecessary since a duplicate already exists.