list1 = [1, 2, 3, 4, 5, 6, 1, 2, 3]

list1.remove(1)   # remove first occurence of element 1 

print(list1)

list1.remove(2)   # remove first occurence of element 2

print(list1)

# list1.remove(100)  # an exception is raised, as value 100 doesn't exists in the list