[tablespoon]

> Except you often need to rewrite them, so now you've got two places (per 'unit') where you could have introduced a bug.

That's not a bad thing, though. DRY might be fine for your main implementation, but redundancy is a time-tested way of catching errors (a.k.a. "double checking").

[h0l0cube]

In theory, if adequate attention is spent on both maintaining the implementation as well as the tests, this is perfectly valid. In practice, this trade-off between expedience and verification goes towards the former when it comes to rapid development.