PostgreSQL Duplicate key value violates unique constraint

Duplicate key value violates unique constraint

Customers table

IdFirst_nameLast_name
1TomGreen
2JacobLogan
3JohnPope
insert into customers
(first_name, last_name) 
values ('John', 'Pope');

ERROR: duplicate key value violates unique constraint
This error occurs when because on table customers
(columns: first_name, last_name) is defined
an unique index and records already exist in the table.