Both of my dulcimers came with the two-course melody, or noter, string, making it a four-string dulcimer. When I'm playing the one I tune to D-DD and use a noter on, the doubled string is okay. But the one I fret with my fingers and tune variously to DAD, DAA, DGD, etc. I found the doubled string didn't add much in the way of volume or clarity of melody notes, so I pulled it off.
Most of the players I know agree that once you've gone to the single melody string, you'll never be comfortable playing doubled again.
Then there're the 4 equidistant stringed dulcimers...