can't check condition in lowercase

Second if-string is't work correctly,what's wrong? and WHY wrong?

oldusers = ['frank', 'Arnold', 'rose', 'john', 'saymon', 'michael', 'albert', 'admin']

newusers = ['albert', 'arnold', 'chris', 'jenny', 'harry']
for user in newusers:

 if user in oldusers:
    print(user + ' must create new name ')

#this line

 if user.lower() in oldusers == user in newusers:
    print(user + ' name already using')

 elif user not in oldusers and user not in newusers:
    print(user + ' availible to use')