Insert Into Tabel Based On Values In The Table!

Hi i am trying to create an insert statement that will insert rows into a table based on the information in the table already.
the table looks like this

Groupid field1 field2
-1 100 200
-1 100 300
-1 300 500
-1 300 600
-1 400 100


the insert looks like this

INSERT Into table1(groupid,field1,field2)
select -1,@passedvalue,field2
from table1
where field1 = @passedvalue1

assume @passedvalue = 700, @passwedvalue1 = 100
Now this is fine however i cannot have a duplicate key (key is comibantion of all 3 fields) thus the first time this runs it works however if it runs again it fails - how can i change the where clause to ignore rows that already exist?
eg if @passedvalue = 300 and passedvalue1 = 500