Try not to duplicate measures (e.g., two different self-report measures of depression) just for the sake of having multiple measures; select measures that use different measurement methods, that...

Try not to duplicate measures (e.g., two different self-report measures of depression) just for the sake of having multiple measures; select measures that use different measurement methods, that measure different targets, that measure different dimensions of the same target, or that are based on information obtained from different sources.