Duplicate keys not caught by `cfn-cli validate`

We were getting stabilization errors on an `AWS::CloudFormation::ResourceVersion` of ours. The `schema.json` passed validation and generated correctly via `cfn-cli`.

Anyway, here is the diff of the change to our `schema.json` that fixed the CloudFormation stabilization issue. I'd expect `cfn-cli validate` to have caught this, but it didn't. As you can see there were two `Indexes` keys defined in the properties of an object definition.