find duplicated record by first and last name - sql

Question

I have a table called beneficials. Some facts about it:
A beneficial belongs to one organization
An organization has many beneficial
Beneficials have first and last names and no other identification form.
Some sample data from the table
| id | firstname | lastname | organization_id |
|----|-----------|----------|-----------------|
| 1 | jan | kowalski | 1 |
| 2 | jan | kovalski | 3 |
| 3 | john | doe | 1 |
| 4 | jan | kowalski | 2 |
I want to find if a beneficial from an organization is also present in other organizations through first and last name and if so, I want to get the organization or organizations ids.
in the sample data above, what I want is given organization id 1, the query should return 2 because jan kowalski is also beneficial on organization 2 but not 3 because even though they match the first name, they don't match the last name
I came up with the following query:
with org_beneficials as (
select firstname, lastname from beneficials where organization_id = ? and deleted_at is null
)
select organization_id from beneficials
where firstname in (select firstname from org_beneficials)
and lastname in (select lastname from org_beneficials)
and deleted_at is null
and organization_id <> ?;
it kinda works but returns some false positive if beneficial from different organizations share the same first or last name. I need to match both first and last names and I can't figure out how.
I thought about joining the table itself but I'm not sure if this would work since an organization has many beneficials. Adding a column like fullname is not something I want to do it here

Answer 1

You can group by first and last names, then filter for duplicates
SELECT firstname, lastname
FROM beneficials
GROUP BY firstname, lastname
HAVING COUNT(*) > 1;
After your edits, it seems you want to select the records of people of a given organization that also appear in a different organization
SELECT *
FROM beneficials a
WHERE a.organization_id != 1
AND EXISTS (
SELECT 1
FROM beneficials b
WHERE a.firstname = b.firstname
AND a.lastname = b.lastname
AND b.organization_id = 1
);