SQLite: updating integer with the value 1 to integer = integer + 1 results in 3 instead of 2

     void OnCollisionEnter2D(Collision2D collision)
     {
         if (collision.gameObject.tag == "item")
         {
             IDbConnection dbcon = new SqliteConnection(connection);
             dbcon.Open();
 
             IDbCommand cmnd1 = dbcon.CreateCommand();
             cmnd1.CommandText = "UPDATE inventory SET amount = amount + '"+collision.gameObject.GetComponent<item_info>().total+"' WHERE NAME = '"+collision.gameObject.GetComponent<item_info>().itemName+"'";
             int success = cmnd1.ExecuteNonQuery();
             Debug.Log(cmnd1.CommandText);
             Debug.Log("returns " +cmnd1.ExecuteNonQuery());
 
             if(success == 0)
             {
                 IDbCommand cmnd = dbcon.CreateCommand();
                 cmnd.CommandText = "INSERT INTO inventory (name, player, world, inventory, positionx, positiony, positionz, amount, hotslot, slot) VALUES ('"+collision.gameObject.GetComponent<item_info>().itemName+"', 'false', 'false', 'true', '0.0', '0.0', '0.0', '"+collision.gameObject.GetComponent<item_info>().total+"', '-1', '-1')";
                 cmnd.ExecuteNonQuery();
                 Debug.Log(cmnd.CommandText);
             }
 
             dbcon.Close();
             Destroy(collision.gameObject);
         }
     }

This is suppose to create a new row whenever colliding with a gameObject with the tag item and update the row in case it already exists, the latter does not work correctly.

Expected result: Have amount increased by whatever collision.gameObject.GetComponent<item_info>().total is. So if the value of amount is 1 and the value of collision.gameObject.GetComponent<item_info>().total is 1 it should now become 2, instead it becomes 3

I don't get it, OnCollisionEnter2D isn't being called twice nor are the database queries. What am I missing here? @_@