Email deduplication is the process of finding records that point to the same email address and keeping one of them, so each mailbox appears once in a list or a send.
Why it matters for outbound
Duplicates waste sending volume and annoy the people you write to. A prospect who gets the same opener twice in one day has a reason to report it as spam.
They also split your data. Bounce, reply and opt-out history sits on separate rows, so a suppression applied to one copy leaves the other live. Merge lists from several sources and the overlap is usually where this happens.
Match on the normalized address, not the raw string. Do the dedupe after you combine sources and before you verify, so you do not pay to check the same address twice.
Example
You merge two exports into one file:
Jane.Doe@Acme.com
jane.doe@acme.com
jdoe@acme.com
jane.doe@acme.comA raw string match finds no duplicates, because case and a trailing space differ. After you trim whitespace and lowercase the address, rows 1, 2 and 4 are the same mailbox. Keep one.
Row 3 stays. It may be the same person on a second mailbox, but nothing proves it, and deleting it loses a real contact. Handle it as a person-level match, with a rule you choose in advance.
Lowercase the whole address for matching. RFC 5321 (September 2026) technically allows a case-sensitive local part but discourages relying on it, and mail systems treat the domain as case-insensitive. Do not strip dots or plus tags from the local part unless you know the provider ignores them.
In LeadOcean data
LeadOcean has no dedupe filter or endpoint for your own lists. It does dedupe in one place: a POST /v1/exports file is deduplicated by person_id, so the same person is not written twice (openapi.json, September 2026).
That key is the person, not the address. If you combine a LeadOcean export with a CSV from another source, run an address-level pass on the merged file yourself.
Enrichment also resolves a person_id to a duplicate group and answers from one row of it. For about 8% of people the record can carry a different person_id than the one you sent, so keep the id you looked up by (openapi.json, September 2026). Details are in the exports docs (September 2026).
For the wider idea, read what is deduplication. Once the list is clean, see personalizing emails for the next step. Pricing is on /pricing. More guides sit in the deliverability and verification hub.
Related terms
Start with a list that is already deduplicated by person
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