Will turn down a graduate (Master's) program have effect on my application for the same program?
In most programs, each application is judged on its merits. That said, however, it is likely that a little note will be appended to your file to state that you were accepted into the program last year and that you rejected the offer. How this note will be judged will depend on the admissions committee, of course. However, unless you can provide reasonable evidence explaining your rejection of the offer of admission, I would imagine that it would count negatively in your favour. I suggest that you provide this reason in the statement of purpose.

@Buzz has commented that your question about deferred admission is pertinent here. If there is no reason for you to reject the offer outright, then a program that offers you the chance to defer acceptance should be the way to go. This is usually the case for students wishing to do a "gap year thing".