How can I check for duplicates before inserting into a table when inserting by select

Question

How can I check for duplicates before inserting into a table when inserting by select:

insert into table1
select col1, col2 
from table2

I need

Answer 1

insert into table1
select col1, col2 
from table2 
where table2.col1 not in (select col1 from table1)