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You don't ensure somebody, you assure them.
"I ensure you that..." is not proper usage. For example, you could say "I assure you that you're wrong" but not "I ensure you that you're wrong". Instead, you'd say "I'll ensure that you're wrong". Slight and subtle difference in use, but a difference nonetheless.
Just because one word is a synonym of the other doesn't mean they're perfectly interchangeable. For example, "believe" and "affirm" are synonyms. You can say "I believe in God" but you wouldn't say "I affirm in God". Instead, you'd say something like "I affirm God's existence".
Capice?
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