Systematic Reviews should be conducted on novel topics to synthesize evidence on that given novel topic.
It is recommended that authors look for their proposed topic of review prior to commencing the review to ensure a recent (within 5 years) systematic review already exists.
Authors should also look at the PROSPERO registry of systematic reviews to see if one is currently being conducted.
Registries will help you find out what reviews are currently in process or have been recently completed. It is also recommended that you register your review so that others can see your review in process. Certain funding agencies may require registration.
For more information on why you should register, consult Registering a Systematic Review.