If you are referring to eating fruit and plan to eat exactly one of each kind mentioned, then you should use your second sentence: I want to eat a banana, an apple and a grape. If you are referring to eating fruit, but you might eat more than one of each kind mentioned, then you should use the plural forms: I want to eat (some) banana s , apple s and grape s . OR if you are planning to eat one banana, one apple and more than one grape, you could say this: I want to eat a banana, an apple and some grape s .
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