Indeed, the spec states that keys SHOULD be unique (with RFC 2119 meaning) and leaves behavior unspecified in case of duplicate key.

My favourite example of dealing with undefined behaviour is this:

In practice, many C implementations recognize, for example, #pragma once as a rough equivalent of #include guards — but GCC 1.17, upon finding a #pragma directive, would instead attempt to launch commonly distributed Unix games such as NetHack and Rogue, or start Emacs running a simulation of the Towers of Hanoi.[7]

Source: http://en.wikipedia.org/wiki/Undefined_behavior