updating a leaderboard for noughts and crosses game

Question

I have a an issue in updating my dictionary from txt file. when ever a new player score is saved, the file creates a new dictionary. here is my code

def save_score(score):
    filename = 'leaderboard.txt'
    player = input('what is your name')
    if os.path.exists(filename):
        with open(filename, "r+") as leaderboard:
            dictionary = json.load(leaderboard)
            dictionary[player]=score
            json.dump(dictionary,leaderboard)
        print("score"+ " " + str(score)+ " "+ "for" + " " +str(player)+ " "+"saved.")
    return

this is what I get when code is run.

{"Bob": 50,"Jane": 2,"Sarah": 5, "Amelia": 20}

then if a new player score get saved , it creates another separate dictionary.

{"Bob": 50,"Jane": 2,"Sarah": 5, "Amelia": 20} {"Bob": 50,"Jane": 2,"Sarah": 5, "Amelia": 20, "bobby": 20}

I would want the file to have just 1 dictionary and update the player name and score and also add new player in 1 dictionary instead of 2. thanks in advance.