Insert Stored Procedure with WHERE clause - stored-procedures

Question

I have a stored procedure for Oracle 10g that needs to create a new row in the table and not create duplicates.
The table allows duplicates, so long as all columns are not the same. This is because the last two columns can differ in values.
With that being said, when I try to store my procedure I get the following flags:
Line # = 10 Column # = 1 Error Text = PL/SQL: SQL Statement ignored
Line # = 13 Column # = 3 Error Text = PL/SQL: ORA-00933: SQL command not properly ended
The procedure looks fine [given I haven't added a WHERE clause for an insert before like this].
So either my format isn't what it should be or my logic is off.
Whatever the case may be, I have tried finding examples online and on stackoverflow and have fallen short.
Any suggestions on how I should tweak this?
(val_ID tablename.column1%type,
val_cat tablename.column2%type,
val_sub tablename.column3%type
)
AS
BEGIN
INSERT INTO tablename (column1, column2, column3)
VALUES (val_ID, val_cat, val_sub)
WHERE ((column1 != val_ID) and (column2 != val_cat) and (column3 != val_sub));
COMMIT;
END;
I have even removed the "(" in WHERE clause and nothing changed.
UPDATE:
tried the suggestion and all errors are gone [however the record didn't create]
(val_ID tablename.column1%type,
val_cat tablename.column2%type,
val_sub tablename.column3%type
)
AS
BEGIN
INSERT INTO tablename (column1, column2, column3)
SELECT val_ID, val_cat, val_sub
FROM dual
MINUS
SELECT val_ID, val_cat, val_sub
FROM tablename;

Answer 1

The insert statement doesn't have a where clause. You could emulate it, though, by using an insert-select statement:
INSERT INTO tablename (column1, column2, column3)
SELECT val_ID, val_cat, val_sub
FROM dual
MINUS
SELECT column1, column2, column3
FROM tablename;

Answer 2

#Mureinik 's example did negate all my errors; however, when tested it didn't create the new row.
So my current work around will be a query in VB.net checking if the value exists and then implementing a simple insert stored procedure:
//Make select statement and look at table for whether more than 0 rows shows up. If 0 rows, then execute stored procedure
If DsAds1.Tables(0).Rows.Count = 0 Then
...do stored procedure
End If
Stored Procedure
(val_ID tablename.column1%type,
val_cat tablename.column2%type,
val_sub tablename.column3%type
)
AS
BEGIN
INSERT INTO tablename (column1, column2, column3)
VALUES( val_ID, val_cat, val_sub);
COMMIT;
END;