I've never heard the phrase "double-hitting" before, but I know it gets people hired. Fire off your resume and cover letter via E-mail, and it's easy to be skipped over. Following up with a paper copy of your resume and cover letter is a way of showing a healthy interest in the job.
From the NYTimes:
Attach a handwritten note that says, “Second submission; I’m very interested,” Ms. Piotrowski said. “I’ve had clients double their rate of interviews simply from doing that,” she said.
Ms. Enelow calls this “double-hitting,” and says she has seen it work remarkably well.
I know someone who got hired by walking into an office with a hard copy of her resume after she'd E-mailed one first. (She also dressed professionally for the occasion, and researched the company thoroughly.)
One expert reported in the NYT story that "a senior-level client of hers got an interview and was hired because the hard copy of his cover letter and résumé reached the company president, whereas his electronic application was rejected by someone in human resources because it did not meet certain rigid criteria."