SQL 2012 :: Checking Existence Of Row Prior To Insert?

I have a situation where our stored procedure inserts records from table_1 to table_2 when they don't already exist (uses the EXIST statement) on that table. If table_1 contains multiple records that are the same, it appears after the 1st record has been inserted, it does not recognize it as being there when it checks the existence when attempting to insert record 2.

Here's an example of the script:

insert into table_2 (col1,col2,col3)
select col1,col2,col3
from table_1 t1
where not exists (select '1'
from table_2 t2
where t1.col1 = t2.col1
and t1.col2 = t2.col2
and t1.col3 = t2.col3)

Data from Table_1 -- Assume that table_2 does not contain these records

col1 col2 col3
AA 11 A1
AA 11 A1
BB 22 B2

All 3 records would be inserted to table_2 in this example.