lst = [ 3, 6, 9, 12, 3, 30, 15, 9, 45, 36, 12, 12] dupItems = [] uniqItems = {} for x in lst: if x not in uniqItems: uniqItems[x] = 1 else: if uniqItems[x] == 1: dupItems.append(x) uniqItems[x] += 1 print(dupItems)