In Chapter 5 of Dan Brown's The Lost Symbol, Katherine receives a call from Mala'kh asking to meet her "again" and telling her that what your brother believes is hidden here in D.C... Katherine seemed to know him, but there is nothing written in this regard in later part of the book. How has Katherine known him before?
The character who calls himself Mala'kh in Dan Brown's The Lost Symbol actually has multiple identities. A second identity he takes on is that of Dr. Abaddon in which he poses as a psychiatrist who pretends to be seeing Peter Solomon, Katherine's brother, as a patient. While the sentence in question may simply be an example of cryptic wording, it is true that Katherine has met with Mala'kh on multiple occasions under at least two different identities.
In Chapter 5, Katherine is heading into her secret lab in which she is studying Noetics and is about to hold an important meeting with someone, as we can tell by her discussion with the security guard. While she is anticipating the meeting, she also receives a phone call, and based on her response to the caller ID, the reader can presume the caller is the same person she anticipates a meeting with. The reader also knows the caller is Mala'kh, and it is at this point that Mala'kh says to her, "We need to meet again," followed by, "That which your brother believes is hidden in D.C....? ... It can be found" (28). We don't really know what Dan Brown means by the word "again," as it can carry a couple of different meanings. We do, however, know that if the person she is speaking to on her cellphone is the one she meant to meet in her lab that day, then it's possible Dan Brown only meant for the word "again" to mean "at a later time" rather than "once again" because the person she is supposed to be meeting is clearly not in her lab. What's also particularly important for the reader to realize is that we don't know what identity Katherine knows Mala'kh by.
Later, on page 113, Katherine cle begins to become involved with Mala'kh's second identity when she receives a phone call on her private cellphone line from a Dr. Abaddon who informs Katherine he is her brother's psychiatrist. Feeling worried, Katherine insists on meeting Dr. Abaddon. The whole meeting is a ruse of Mala'kh's to find out what Katherine knows about the Freemason pyramid he is looking for that he believes will give him special powers. On page 113, we also realize her meeting with Dr. Abaddon is actually told as a flashback, as indicated by the sentence, "Although she had met this man for the first time only this afternoon, he had earned her trust in a matter of minutes. Completely." There is also a reference to Sunday afternoon, and we know from Chapter 5 that Katherine was supposed to meet with that mysterious someone on a Sunday.
Hence, due to the flashback, the reader can presume that the person Katherine is on the phone with in Chapter 5 is actually Mala'kh under the identity of Dr. Abaddon, and, yes, she had met him earlier that day at his home.