я бы сделал итератор - самый правильный
pythonic way
def flt(a):
deleted = []
v = None
n = 0
for x in a+[None]:
if not v:
v = x
n = 1
continue
elif v == x:
n += 1
continue
elif n in [1,2]:
yield v
if n == 2:
yield v
else:
deleted.append((v,x))
v = x
n = 1
print(f'deleted={deleted}')