Generally, we do not recommend that you export references directly from a database into Covidence. Citation managers such as SciWheel and Endnote have much more powerful deduplication features, so putting your citations in a reference manager will make it easier to remove all duplicates before you import citations into Covidence.
If you do need to export references directly from a database into Covidence, the instructions on the Preparing Your References for Covidence page will show you how.