A true duplicate is an item that have a same sitelink with another. All Wikidata sitelinks are stored in the
wb_items_per_site table which has an unique composite index on site ID and sitelink, and all edits that affect sitelinks will result in an update in the table. Therefore, adding the same sitelinks to multiple items will usually be prevented. However, the table may fail to include all sitelinks of all items, because of database error or software bugs. In this case, it is possible for two items to have same sitelinks.
How to fixEdit
See also: Help:Merge
Items with a single sitelink:
- click on the first item
- click on the sitelink to open in a new tab
- click on the link "Wikidata item": you should be on another item
- select for merging (with the merge-gadget)
- return to the tab with the first item, select ("process the postponed merge")
If this does not work, you may try to merge them using Special:MergeItems. If not working either, remove duplicated sitelinks from one of the item and retry.