Well, if you have conditional logic set for the two fields so that only one of them is shown then the value for the image field should not be submitted at all. So I can’t say why this would be happening. The only reason that both fields would be submitted is if both fields are visible, or at least that’s the way it’s supposed to work.
ACF saves values in the order that they’re submitted, which is usually the order that they appear in on the page.
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