Duplicate key value violates unique constraint

Duplicate key value violates unique constraint

Customers table

Id First_name Last_name
1 Tom Green
2 Jacob Logan
3 John Pope
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.