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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def check_unique_tokens(sender, instance, **kwargs):
 """ Ensures that mobile and email tokens are unique or tries once more to generate. """

    if isinstance(instance, CallbackToken):
        if CallbackToken.objects.filter(key=instance.key, is_active=True).exists():
            instance.key = generate_numeric_token()