What about this?
double[] v = Enumerable.Repeat(x, n).ToArray();
EDIT: I just did a small benchmark; to create 1000 arrays of 100000 elements each, using a loop is about 3 times faster that Enumerable.Repeat
.
Repeat
00:00:18.6875488
Loop
00:00:06.1628806
So if performance is critical, you should prefer the loop.