“Non mais” should obviously not be translated literally. It replaces a WTH (or WTF, or “Wait!”…) in the sense of “why on Earth would that happen/be?”.
It could be replaced with “Attends !…” (literal translation of “Wait!”), or “sans déconner” (literal translation of “No kidding!”) like in:
Attends/Non mais/Sans déconner ! Tu es en train de me dire que…
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