I have two forms: Username and Password. Under these forms, I have a "Login" button. Do I need a title for the Login Page? Can the user understand the button? Otherwise, I would have two sections saying "Login" - title and button.
It's okay (and often desirable) to repeat the word login.
Typically at the top of the form (aka entry point) you will have a prompt that tells the user what the workflow is: "Please log in"
It's OK to have the button at the bottom say Log in again.
There is a difference in function between #1 and #2. The prompt tells the user what to do, and the button allows the user to perform that exact action (ie you asked me to log in, and I'm now logging in): this is good practice for a clear UX workflow.
Remember that writing UX is different from writing an article or an essay. Word redundancy is inelegent in an essay, but it serves a clear design purpose in UX!