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Current designs may contain several identical copies of the same circuit (or functional unit). Such circuits can be tested by comparing the output vectors they produce under identical input vectors. This alleviates the need to observe the output response, and facilitates online testing. We show that testing of identical circuits by output comparison can be done effectively even when the input vectors applied to the circuits are not identical. This allows concurrent online testing even when the circuits are not driven from the same source during functional operation. We investigate several issues related to this observation. We investigate the use of both structural and functional analysis to identify situations where nonidentical input vectors can be used for fault detection based on output comparison. We also consider the use of observation points to improve the fault coverage. We present experimental results to support the discussion and the use of nonidentical input vectors for concurrent online testing of identical circuits.