How to delete all instances of a repeated number in a list?

You can always have two sets. One to check if seen and another one to keep unique only. set.discard(el) will remove if exists.

Inputlist = [2, 3, 6, 6, 8, 9, 12, 12, 14]

seen = set()
ans = set()

for el in Inputlist:
    if el not in seen:
        seen.add(el)
        ans.add(el)
    else:
        ans.discard(el)

print(list(ans))

EDIT: for giggles I measured the performance of these two solutions

from timeit import timeit


first = """
def get_from_two_sets():
    seen = set()
    ans = set()

    for el in (2, 3, 6, 6, 8, 9, 12, 12, 14):
        if el not in seen:
            seen.add(el)
            ans.add(el)
        else:
            ans.discard(el)"""


second = """

def get_from_counter():
    return [el for el, cnt in Counter((2, 3, 6, 6, 8, 9, 12, 12, 14)).items() if cnt == 1]
    """


print(timeit(stmt=first, number=10000000))
print(timeit(stmt=second, number=10000000, setup="from collections import Counter"))

yields

0.3130729760000577
0.46127468299982866

so yay! it seems like my solution is slightly faster. Don't waste those nanoseconds you saved!

@abc solution is clean and pythonic, go for it.

A simple list comprehension will do the trick:

Inputlist = [2, 3, 6, 6, 8, 9, 12, 12, 14]
 
Outputlist = [item for item in Inputlist if Inputlist.count(item) == 1]

You can use a Counter

>>> from collections import Counter
>>> l = [2, 3, 6, 6, 8, 9, 12, 12, 14]
>>> res = [el for el, cnt in Counter(l).items() if cnt==1]
>>> res
[2, 3, 8, 9, 14]