How do I tune my dulcimer?

I recently inherited a dulcimer. I’d love to play it. I have a little group that gets together to play once in a while, and a dulcimer would fit in nicely. The dulcimer strings I bough indicate CCCG, but I’m pretty sure the instruction book I have says DDDA. Of course, I have no idea which D. When I tried to tune it to D, I broke the string. Are the two strings next to one another the same note? I don’t understand what the point of that would be.

The third and fourth (treble) strings are considered double melody strings, tuned to the same note. It adds some oomph to the treble.

This site looks like it has a lot of helpful information. Neither it nor several others I saw from Googling mountain dulcimer tuning showed DDDA (bass to treble). The variations I ran across include DAAA, DADD, DAGG, and DACC. I would suggest check all the pages that mention “tuning” to get an idea of what is normally done.

The C-G tuning as opposed to the D-A tuning might be an option to match a voice or match other instruments playing in the key of C rather than D. Offhand I would expect that a set of dulcimer strings could adapt to either C tuning or D tuning, but I’m not sure. It is conceivable that a particular set of strings is designed for C rather than D, and gets overstretched tuned up to D.

ETA: DAAA tuning is probably normally referred to as DAA, and DADD as DAD, etc., with it being understood that the fourth string always takes the same note as the third string.