Sorry David, I have to disagree. If you have dimension A (width) and dimension B (height), then you have completely defined a rectangle and any other dimension is overconstrained, unless it's a reference dimension. In this scenario, while a relation may make A = 2*B, it doesn't factor into anything. You could comment it out and it wouldn't change anything. It is geometrically overconstrained, and you must either make the diagonal the reference dimension, or one of the other 2.
I'll have to disagree with Doug (just because I'm that retentive 
) and say that he's right....except about the weak dimension. A strong or locked dimension will make it overconstrained, a weak dimension will disappear as soon as the sketch is fully constrained.