A scanner without double-sided gives you two files: the stack, then the stack flipped over. Drop both in and the pages go back one from each.
What it does not do: If the backs are in reverse order, reverse them first: this tool interleaves, it does not guess the direction. And a single file holding the fronts then the backs is handled by “Reassemble a double-sided scan”.
Nothing is uploaded. Cut your network connection once this page has loaded: the tool keeps working. How to check it yourself.