# data
result_years_list = [2011, 2011, 2011, 2012, 2012, 2012, 2012, 2012, 2012, 2012, 2012, 2012, 2012, 2012, 2012, 2013, 2013, 2013, 2013, 2013, 2013, 2013, 2013, 2013, 2013, 2013, 2013, 2014, 2014, 2014, 2014, 2014, 2014, 2014, 2014, 2014, 2014, 2014, 2014, 2015, 2015, 2015, 2015, 2015, 2015, 2015, 2015, 2015, 2015, 2015, 2015, 2016, 2016, 2016, 2016, 2016, 2016, 2016, 2016, 2016, 2016]

# solution 1
years_set = set()

def list_hack(value):
    years_set.add(value)
    return value

[list_hack(year) if year not in years_set else '' for year in result_years_list]

# solution 2
class CustomSet(set):
    def __init__(self):
        self.set = set()

    def __contains__(self, key):
        return key in self.set

    def __str__(self):
        return self.set

    def add(self, value):
        self.set.add(value)
        return value

set_redux = CustomSet()
[set_redux.add(year) if year not in set_redux else '' for year in result_years_list]


# solution 3
years_set = set()

def list_hack(value):
    years_set.add(value)
    return value

custom_gen = (list_hack(year) if year not in years_set else '' for year in result_years_list)

[i for i in custom_gen]