Create non-repeat username data base

I wrote def addmember to add member to database.db. I want no repeat in user name, example: when i add member (1,1) to database, i cannot add member (1,2) to my database because already have user with username "1". But my code still allow it ! Can some one help me ? Node that self.textEdit.toPlainText(), self.textEdit_2.toPlainText() is just my qtextedit object, dont mind it

def addmember(self):
        self.connection = sqlite3.connect("database.db")
        cur = self.connection.cursor()
        cur.execute("CREATE TABLE IF NOT EXISTS USERS(USERNAME TEXT ,PASSWORD TEXT)")
        cur.execute("SELECT * FROM USERS WHERE USERNAME=?", (self.textEdit.toPlainText()))
        for row in cur.fetchall():
            print(row)
        if (len(cur.fetchall())==0):

            cur.execute("INSERT INTO USERS VALUES(?,?)",(self.textEdit.toPlainText(), self.textEdit_2.toPlainText()))
            self.connection.commit()
            self.textEdit.clear()
            self.textEdit_2.clear()
            cur.close()
        else:
            self.textEdit.clear()
            self.textEdit_2.clear()
            cur.close()
        self.connection.commit()
        self.connection.close()

Your calling fetchall twice. The first fetches all the rows. The second will never get any rows and the len will always be 0 so your insert will always happen.