I think the two sentences are not a similar case because this depends on the kind of verb you are using i.e. we usually use the verb 'forget' in the past form if we are talking about a particular state of forgetting something. on the other hand we usually use the verb' know' in the present form when we say we already know something
so using the past perfect is correct in this sentence " I forgot " --I hadn't forgotten ....whereas " I don't know "--I knew
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