State department is really put out that he has to do this..
Check it out:

Secretary of State John Kerry has agreed to testify on the Benghazi attack in response to the second subpoena from the House Oversight Committee. Furthermore, the State Department reasoned that if Kerry speaks before the House Oversight Committee, there will be no need for him to testify again before the Special Committee on Benghazi.

As captured by the Associated Press:

In a letter to Chairman Darrell Issa, a clearly annoyed department said diplomatic responsibilities prevent Kerry from testifying on May 29, the date of a committee subpoena. The department offered alternative dates of June 12 or June 20.