Hello, it depends of the context. You would use "I saw him once" if it happened at a precise moment such as "yesterday, the last year" etc...
Otherwise if you say it like a "result" of something for example: "I've seen him once, I feel so sad."
April 1, 2015
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They're both correct, but they have slightly different connotations.
If you're interesting in knowing the difference, just ask again.
April 1, 2015
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