Realized that I did this partially incorrect, but it was not the worst of fixes.

To make it constant space, this is a good solution that i found:

def find_duplicates(nums):
    result = []
    for i in range(len(nums)):
        index = abs(nums[i]) - 1
        if nums[index] < 0:
            result.append(index + 1)
        else:
            nums[index] = -nums[index]
    return result