Ideally, it's like double-entry book keeping. If you try to express the same thing in two different places, ideally in two different ways, you're less likely t

Ideally, it's like double-entry book keeping. If you try to express the same thing in two different places, ideally in two different ways, you're less likely to make the same mistake in both places than to make any mistake in the first place. This applies both to types and tests.