She didn't look at two piles of money, there was 1 £50 note on the side.
She picked it up, did the clean and when she came back to the side another £50 note had appeared, she picked it up and took it.
So yeah theres the potential she thought 'oh, thought I'd picked that up, clearly not' and pocketed it, and at the point of leaving the the property hadn't realised she'd actually taken it twice.
When doing routine stuff its pretty easy to go into auto pilot and not be sure if you're remembering doing that thing this week, this morning, last week etc.
Case in point, I need my OH to spot me taking my medication each day, because I do it every day, if he doesn't also see it, I can mix up remembering taking yesterdays pills... or I have twice now given myself my long acting insulin twice in one day because I thought I was remembering the previous days jab, not the one I did an hour earlier!
If the OP had left a different set of notes, or in a different place - then yes it would be much clearer!
So I guess its down to how the cleaner behaves now - if they get a text saying 'oops, seems you picked up two payments as I didn't realise husband had left the money out already, keep that for next weeks payment' and they react by denying it... you know!