Migration Problems while creating a new migration file

Hello ,

I am a ROR beginner. I have a controller named “users” and the
corresponding view,model created using the command “ruby script/generate
scaffold user name:string”.By running the command rake db:migrate will
create the table “users” with a field “name” and the migration file
created automatically is “001_create_users.rb”. Now when I tried to add
one more column “password”
by creating a new migration file using the command “ruby script/generate
migration create_users” and made the appropriate changes like
“add_column :users,:password,:string” in that file, an error is occuring
like “Another file named 001_create_users.rb already exists”. May any
one let me know what is the problem? I am using the right way or not?

Thanks in advance