FBI search warrant request and related materials were released by the court. NY Post has posted the complete document set.
The relevant section starts around page 8, whereby the FBI notes that a review of non-content email headers is what triggered the new investigation and associated search warrant.
That non-content header information should have included From and To fields, date and time, and subject. This information was more than sufficient for the FBI to compare these headers with the headers of previously investigated emails and to determine that the emails were on Weinger's laptop because of automatic backup procedures.
The FBI personnel investigating these emails should have had the knowledge necessary to do spot checks of the headers and should have known the emails were copies of ones they've already seen.
That they didn't do so either demonstrates incompetence or a deliberate decision to find some reason to re-open the Clinton email investigation in the crucial last days of the election. As Chris Tognotti of Bustle notes:
Also notable, the FBI specifically referenced the "confidential nature of the investigation" in the filing, despite the fact that it was made days after the release of Comey's letter. Suffice to say, it didn't exactly seem "confidential" by that point. Obviously, the past is prologue, and nobody will ever know precisely how the 2016 race would've turned out absent that cache of emails, and Comey's decision, and the search warrant, and all the rest. But it's hard for a lot of people not to wonder, that much is clear.
Huffington Post asked legal experts their opinions on the search warrant request. Needless to say, none of them were impressed.
The legal experts’ argument against the validity of the subpoena boils down to this: The FBI had already publicly announced that it could not prove Clinton intended to disclose classified information. Without that intent, and without evidence of gross negligence, there was no case. The warrant offers no suggestion that proving those elements of the crime would be made easier by searching new emails.
The essence of the warrant application is merely that the FBI has discovered new emails sent between Clinton and Abedin.
That’s not enough.