Removing duplicates

dup_removal.py

def answer(numbers):
    seen = set()
    counter = 0
    zero_counter=0
    unique = []
    duplicate_value=None
    numbers.insert(0,0)
    for x in numbers:
        if x in seen:
            duplicate_value = x
            break
        seen.add(x)
        unique.insert(counter,x)
        counter+=1
    if numbers[-1]!=0 and duplicate_value is None:
        return 0
    if numbers[-1]==0 and duplicate_value is None:
        return len(numbers)
    elif counter ==len(numbers):
        return 0
    start_loop=unique.index(duplicate_value) 
    if duplicate_value == 0:
        start_loop+=1
    return counter-start_loop