This is a grammatical error involving the use of incorrect verb tense. Specifically, it is an error in forming the past perfect tense.
People often make this error due to confusion between "know" as a present or past simple form and "known" as the past participle. The structure of past perfect tense requires the past participle form, not the bare infinitive or present form of the verb.
The correct form is "had known," where "had" is the auxiliary verb and "known" is the past participle.
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