The expression BELIEVE YOU ME is used for emphasis in phrases that express certainty, surprise, annoyance, etc.:
Believe you me, she can really do it, even if she says she can’t.
We’re doing the right thing, believe you me.
This product, believe you me, is one of the best on the market now.
Well, believe you me, I will never forget what you called me.
And, believe you me, my offer is much better than the other.
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