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I have made up the example below.

(1) I said to John, "I'll lend you my calculator. Please take good care of it. I've heard that Bill likes to steal other people's calculators and resells them."

John replied, "Don't worry. I'll make sure that I won't let him steal yours."

My non-native English speaking friends think it's grammatically wrong to use the future tense twice in the same sentence. Is my last sentence really wrong? Thank you very much.

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ansonguyJohn replied, "Don't worry. I'll make sure that I won't let him steal yours."

Grammatically correct, but unnatural.

I'll make sure that he doesn't steal yours. (future, then present)

CJ