MINOR: cache: Reject duplicate cache names

MINOR: cache: Reject duplicate cache names

Using a duplicate cache name most likely is the result of a misgenerated
configuration. There is no good reason to allow this, as the duplicate
caches can't be referred to.

This commit resolves GitHub issue #820.

It can be argued whether this is a fix for a bug or not. I'm erring on the
side of caution and marking this as a "new feature". It can be considered for
backporting to 2.2, but for other branches the risk of accidentally breaking
some working (but non-ideal) configuration might be too large.