If else in stored procedure sql server

Question

I have created a stored procedure as follow:

Create Procedure sp_ADD_USER_EXTRANET_CLIENT_INDEX_PHY
(
@ParLngId int output
)
as
Begin
    SET @ParLngId = (Select top 1 ParLngId from T_Param where ParStrNom = 'Extranet Client')
    if(@ParLngId = 0)
        begin
            Insert Into T_Param values ('PHY', 'Extranet Client', Null, Null, 'T', 0, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, 1, NULL, NULL, NULL)
            SET @ParLngId = @@IDENTITY
        End
    Return @ParLngId
End

So I set a variable called @ParLngId, check to see whether a table contains such data, and if it does, return the value; otherwise, I insert one and return the variable containing the Id of the inserted line. However, it now displays a SqlException:

Subquery returned more values. This is not permitted when the subquery follows =,! =, <, <=,>,> = Or when used as an expression.

Does somebody have a solution?