The uniqueness (or otherwise) of test outputs ought to have a bearing on test effectiveness, yet it has not previously been studied. In this paper we introduce a novel test suite adequacy criterion based on output uniqueness. We propose 4 definitions of output uniqueness with varying degrees of strictness. We present a preliminary evaluation for web application testing that confirms that output uniqueness enhances fault-finding effectiveness. The approach outperforms random augmentation in fault finding ability by an overall average of 280% in 5 medium sized, real world web applications.