Whether you are updating a review with new database search results, or adding studies from other sources (grey literature or citation tracking, for example) it's an easy process in Covidence.
You can import new studies into your review at any time. You can also choose whether to add them to the Title and abstract stage, Full-text review, or straight to Extraction.
Covidence automatically de-duplicates upon import. The de-duplication mechanism checks both within the file being uploaded as well as against all previous imports, regardless of what section of the review the references have been moved to. 
We recommend documenting the status of numbers in the PRISMA flowchart before importing the new studies. You can export the information at any time and log it elsewhere for reference. To do this, select PRISMA in the top right of screen on the Review summary page and then select Export data. This gives the numbers as text that you can copy into another file.
Alternatively, you can create a new review and import the new references into this review. This will treat the import separately from the previous work done. If you decide to create a new review, please note that you will need to configure your review settings and set up the extraction templates and forms again.