How can I check whether a paper I found is a duplicate of one already in my library?

Compare the DOI first and the title second against your existing entries, then merge the two records if they match and keep the version with the most complete metadata. This two-field check catches most duplicates because a DOI resolves to the current information supplied for the identified object, while the title catches items saved without a DOI.

1. Look up the DOI. Open the paper's landing page and copy the DOI exactly as printed. A DOI name resolves through the Handle System to the current information supplied for the identified object, commonly a landing page maintained by the registrant, so the landing page is the place to confirm which record the DOI points to.
2. Search your library for that DOI. If an entry already carries the same DOI, you have found a duplicate. If the DOI returns no match, move to the title check before concluding the item is new.
3. Compare the title. Search your library for the paper's title, including subtitle. A match on title with no DOI in either record is a likely duplicate; confirm by checking that authors and publication year agree.
4. Run duplicate detection. Zotero identifies potential duplicate items and provides a merge interface that preserves collections and tags while combining item records. Use it to review the pair side by side rather than deleting one entry by hand.
5. Merge and keep the fuller record. When the merge interface combines the records, keep the field values from the version with the most complete metadata, such as a full author list, volume and page details, and an abstract.

A DOI mismatch does not by itself prove the items are different: the same paper can appear under a preprint DOI and a published-article DOI. Treat that case as unresolved and compare the title, authors, and year before deciding whether to merge or keep both records with a note on the relationship.

For metadata you want to verify before merging, a DOI lookup through the Crossref REST API returns the metadata deposited for that DOI, which you can compare against the fields in your library entry.