Mendeley usually syncs Desktop and Online components when you exit the account, but in cases where this isn't automatic, you can also manually sync.
Think you might have loaded a resource more than once? Let Mendeley help you find it and merge the records.
Use Mendeley's Merge Feature
Manually Merge Documents
Sometimes a resource is missing details (authors, dates, abstracts, etc.). Mendeley has Needs Review to help manage these. Click on Needs Review to edit & manually add the missing details.