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From http://en.wikipedia.org/wiki/Repeat_sign
When a repeat calls for a different ending, number brackets above the bars indicate which to played the first time (1), which to play the second time (2), etc. These are called "first-time bars" and "second-time bars," or "first and second endings."
Is there a case when you can only use one of them?
By definition, there isn't. You only need the first-time and second-time bars when you have two different endings.
When you don't have different endings, you don't need to use either first-time or second-time bars. When you do have at least two different endings, you must use both bars. There may be more, too -- some pieces have a common repeated part and more than 2 different endings.
So, it's a "both or nothing" kind of deal.
if the music in the last bar/s of the phrase is identical, you'd use a repeat barline at the end of the phrase, but not put the 1st-time line over the last bar/s. The performer would play the whole phrase identically twice (in the absence of further performance instructions such as 'first time f, second time p',) including the final bar/s.
