Searching for registered, in-progress and published systematic reviews before starting a new one is a standard, endorsed by Cochrane Collaboration, Joanna Briggs Institute, 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 in-progress systematic reviews include PROSPERO International, Cochrane Database for Systematic Reviews (CDSR) and 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 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 with all the Evidence Based Medicine filter.