The story of destiny is actually cycular. Just like Dark Souls the events in destiny repeats over and over, this can be seen in the opening cinematic where you see the mountains changeling. This explains why we can kill bosses over and over, its the cycle repeating.
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Or they just walked really far so the scenery changed.
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Nope different versions of the same mountain, definitely so Why the stranger pops in and out of time, she has to visit another guardian
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Or they are different mountains. Or that she just had to go somewhere else.