The result is 2 executions of the method body (where in my opinion, only 1 is expected) and 2 cache entries (again, only 1 is expected).
The result of this problem is that methods with array type parameters will always result in a cache miss (the body is always executed) and a cache put is always done after the method is executed (filling the cache with garbage that's never used again). This second part is particularly troublesome, as useful stuff ends up getting evicted as the cache fills with garbage.
The problem is that DefaultKeyGenerator (Spring < 4) and SimpleKeyGenerator (Spring 4 and later) use hashCode(), and the hashCode of an array isn't specifically defined, and bubbled up to Object.hashCode() which uses the memory address of the array. Since each array, even if it has the same items in it, has a different address, the hashCode is always different.
Changing the implementation to use deepEquals/deepHashCode fixes this problem.
Fixed the plain array parameter case for both SimpleKeyGenerator (wrapping it with a SimpleKey) and DefaultKeyGenerator (falling back to hash calculation) in master now. The latter will be backported to 3.2.9 soon.
Note that the 3.1.x line has been retired already, so we'll only fix this for 4.0.3 and 3.2.9. We strongly recommend an upgrade to 3.2.8+ in general.
This is available in the latest 4.0.3 snapshot already, and will be available in the immediately upcoming 3.2.9 snapshot as well (see http://projects.spring.io/spring-framework/ for Maven coordinates). Please give it a try and let us know whether it works for you...