When you export sources from multiple databases, you will frequently end up with duplicate copies of the same source. To help you get these out of your library, Zotero will automatically find duplicates for you. It does so on the "My Library" level; it doesn't locate duplicates in individual folders. It also does not affect articles that you've copied into multiple folders. If you want to keep track of how many duplicates you're removing for a project (such as for a PRISMA flowchart), you should create a new group and move your sources there for a "clean" library in which to deduplicate.
Under each library you have in your Zotero desktop program (either your personal library or your group libraries), you'll see a folder titled Duplicate Items.
If this folder is empty, your library doesn't include any duplicates. If it isn't, Zotero requires you to review and merge each duplicate entry individual. (Unfortunately, there is not automatic deduplication tool.) To do so, click on a title in the center pane of your Zotero program, and Zotero will highlight the other title(s) that are duplications of that source.
You can compare the sources, and choose the version to keep on the right pane. Click Merge Items when you're ready.
Unfortunately, Zotero might not be able to identify all the duplicates in your library. I recommend spending a few minutes skimming through your library to see if you spot any yourself. If you do, select the two articles, then right-click over them in the center pane and choose Merge Items.
Just like before, it will give you a choice on which one to keep.
Because this process requires manually clicking through each set of duplicates, if you have thousands of duplicates and access to EndNote, I recommend exporting your library into EndNote, completing the deduplication there, and then importing the deduplicated library back to Zotero.