How to avoid decorating a function with the same decorator again in Python

ignoring_duplicated_decorator.py

def dec(func):
    _decorators = getattr(func, '_decorators', [])
    if dec in _decorators:
        print('Ignoring...')
        return func

    _decorators.append(dec)

    def inner(*args, **kwargs):
        print('Calling...')
        return func(*args, **kwargs)

    setattr(inner, '_decorators', _decorators)
    return inner


@dec
@dec
def foo():
    print('Foo')


@dec
@dec
@dec
def bar():
    print('Bar')


foo()  # Ignores once
bar()  # Ignores twice