The exclamation mark ends a sentence with emphasis. Usually, ending a sentence once is enough. You use one point, one question mark, one exclamation mark.
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Of course, if you end every sentence with an exclamation mark, you have to end emphasized sentences with at least two exclamation marks.
Be Sparing of Exclamation Marks
This is why you should
- use the exclamation mark scarcely
and only when you really feel like having that huge exclamation point above you (if you were a cartoon figure, of course).

