Final chapter in the drama: I have found a solution in Go for complex keys in YAML. It's very complex -- basically I had to redo some of the decoding that the YAML library does, but it does work with the public APIs and does emit an error if duplicate complex keys are used in a YAML map.
One thing to note -- most YAML parsers are "non-strict" in that they allow for maps to have repeating keys, in which case later keys tend to override previous ones. Except ... in the case of a YAML merge (<<) in which the spec specifically says that keys do not get overridden.
Does TOSCA specify if duplicate keys are allowed? It's probably a good idea to do so in order to maintain compatibility between TOSCA parsers.