Prevent duplicated value

I have a a table called Process that has columns something like this

Hold, GId, Source, Type, Operation

and Temp_Table that looks something like this

Hold, GId, Source, Type

Hold_Table looks something like this

Hold, GId, Source, Type

I compare Temp_Table and Hold_Table and insert the record into Process table with "Add" Operation if the Source is not found in Hold_Inv table but I'm just wondering how can I prevent it from inserting duplicated value into Process table.

INSERT INTO Process (Hold, GId, Source, Type, Operation)
    SELECT Hold, GID, Source, Type, 'ADD' Operation 
    FROM Temp_Table
    WHERE Source IN (
        SELECT Source  
        FROM (
            SELECT Source 
            FROM Temp_Table Thi
            WHERE NOT EXISTS (
                SELECT 1
                FROM Hold_Table Hi 
                WHERE Thi.Source = Hi.Source
            )
            AND Thi.Source <> 'NOT FOUND'
            AND LEN(TRIM(ISNULL(Thi.Source, ''))) > 0
        ) T1
    )  

I run the query twice then the record from Temp_Table are inserted twice into the Process table.

It should be inserting twice if one column value is different but if they all the same then it shouldn't be inserting twice anymore.

Any help or suggestion would be really appreciated.

First of, your query can be simplified from :

INSERT INTO Process (Hold, GId, Source, Type, Operation)
SELECT Hold, GID, [Source], [Type], 'ADD' Operation 
FROM Temp_Table
WHERE Source IN (
        SELECT Source  
        FROM (
            SELECT Source 
            FROM Temp_Table Thi
            WHERE NOT EXISTS (
                SELECT 1
                FROM Hold_Table Hi 
                WHERE Thi.Source = Hi.Source
            )
            AND Thi.Source <> 'NOT FOUND'
            AND LEN(TRIM(ISNULL(Thi.Source, ''))) > 0
        ) T1
    )  

To :

INSERT INTO Process (Hold, GId, Source, Type, Operation)
SELECT Hold, GID, [Source], [Type], 'ADD' Operation 
FROM Temp_Table Thi
    WHERE NOT EXISTS (
        SELECT 1
        FROM Hold_Table Hi 
        WHERE Thi.Source = Hi.Source
    )
    AND Thi.Source <> 'NOT FOUND'
    AND LEN(TRIM(ISNULL(Thi.Source, ''))) > 0

To exclude insert of duplicates, you can add the target table in the SQL in the NOT EXISTS :

INSERT INTO Process (Hold, GId, Source, Type, Operation)
SELECT Hold, GID, [Source], [Type], 'ADD' Operation 
FROM Temp_Table Thi
WHERE 
NOT EXISTS (
    SELECT 1
    FROM Hold_Table Hi 
    WHERE Thi.Source = Hi.Source
)
AND Thi.Source <> 'NOT FOUND'
AND LEN(TRIM(ISNULL(Thi.Source, ''))) > 0
AND NOT EXISTS (
	SELECT 1 
	FROM Process Pro
	WHERE 
		Thi.Hold = Pro.Hold and 
		Thi.GID = Pro.GID and 
		Thi.[Source] = Pro.[Source] and
		Thi.[Type] = Pro.[Type] 
)