Removing elements that have consecutive duplicates

>>> L = [1,1,1,1,1,1,2,3,4,4,5,1,2]
>>> from itertools import groupby
>>> [key for key, _group in groupby(L)]
[1, 2, 3, 4, 5, 1, 2]

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For the second part

>>> [k for k, g in groupby(L) if len(list(g)) < 2]
[2, 3, 5, 1, 2]

If you don't want to create the temporary list just to take the length, you can use sum over a generator expression

>>> [k for k, g in groupby(L) if sum(1 for i in g) < 2]
[2, 3, 5, 1, 2]

Oneliner in pure Python

[v for i, v in enumerate(your_list) if i == 0 or v != your_list[i-1]]