I had a similar experience with male egotism (or whatever) yesterday. Three guys on BMX-type bikes were strung out along the right side of the towpath. I was pedaling just a tad faster, so as I came up behind the first guy I called out "Passing on the left!" and pulled over. He had a cel phone tucked between his head and his shoulder and totally ignored me as I went by. As I came up to the second guy, I called out again. As I got up next to him, though, he picked up his pace and rode along next to me. So, I kicked my cadence into high gear and moved ahead and back over in front of him. Then I came up behind the third guy, called out, and the doofus did the exact same thing his buddy had done! So, again, I kicked up the cadence and sprinted away from them, shaking my head in consternation the whole way.

To have someone double up on the trail would really aggravate me, though. In your situation, deena, I'd have been tempted to yell something a little more colorful than "Passing on the left!".