While it may be mean soemthing else to the fiddlers here, the reference to "double stopped" as the term is used by the better amateurs of my acquaintance means that two strings are bowed at the same time, without regard to where the fingers go.

My guess was that the OP meant an open G and an open D bowed at the same time weren't in tune with each other (a true fifth apart).

If it was meant that the G string fingered at D isn't in tune with the open D string (one of many possibilities), we'd have to have a good photo and a micrometer to know whether the G string was just fingered in the wrong place, since on a fretless instrument the right place to finger the G string to get a D is "wherever it's in tune with the other D."

Since fiddle strings are relatively low tension, a little sideways push can "bend" the note off pitch, maybe even easier than on one of those two-by-fours they call "electric guitars."

John