An employer may require that a request for leave under subparagraph (C) or (D) of paragraph (1) or paragraph (3) of section
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(a) of this title be supported by a certification issued by the health care provider of the eligible employee or of the son, daughter, spouse, or parent of the employee, or of the next of kin of an individual in the case of leave taken under such paragraph (3), as appropriate. The employee shall provide, in a timely manner, a copy of such certification to the employer.