Denethor did not actually see Aragorn when he returned to Gondor; he was dead by this time, and we are left to draw our own conclusions about precisely how Denethor knew of Aragorn's imminent arrival. Perhaps Aragorn had aged enough that Theoden did not recognize him, or at least could not immediately place the face. Perhaps Theoden did recognize him; perhaps Aragorn took steps to keep from being recognized.
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