Searching for registered, in-progress, and published systematic reviews before starting a new one is a standard endorsed by the Cochrane Collaboration, the Joanna Briggs Institute, the Institute of Medicine (U.S.), amongst others.  The first step in planning a systematic review is to establish that a new review project is needed and will provide a unique contribution to clinical and policy decision-making. 

The recommended registers to search for in-progress systematic reviews include PROSPERO International, the Cochrane Database for Systematic Reviews (CDSR), and the Joanna Briggs Institute EBP Database.

It is important to note that CDSR and EBP databases include both registered protocols and published reviews.

PubMed and EMBASE both allow users to filter systematic reviews.  To search for systematic reviews in PubMed, use the article type filter on the sidebar, or enter your search terms followed by AND systematic[sb] in the search box.  In EMBASE, limits are displayed below the search bar, with 'systematic review' included within all the Evidence-Based Medicine filter.