etags of resources seem to be the hash of the resource object being returned.
Are they unique?
You should not use the etag as a key. The plan is for the etag to be used as a way of returning data only if its changed since the last time the resource was requested, but we have yet to implement the functionality. Please see http://forum.aws.chdev.org/t/etags-in-documentation-but-not-in-resources/476/2