the key, I think, is not the needless redundancy of the code, but the danger of creating those overlapping conditions.

You're contradicting yourself. :-) Try: "the key is the needless redundancy of the code because of the danger of creating those overlapping conditions". It's the Don't Repeat Yourself principle. The potential for overlapping conditions is just one manifestation of the same basic danger: if you make multiple copies, you risk having them run out of synch. That's why you should always strive to do things Once And Only Once.