Something a little like this happened to me last Christmas - 2 relatives gave me cheques, but I had some surgery last December and wasn't able to get around by myself. For One reason or another, I then forgot about the cheques folded up in a side compartment in my purse.
One relative texted in February to say they'd noticed it hadn't been cashed and should they send another? I apologised profusely and said no, I had it, but that because of all that had happened, getting to the bank was tough, and even when well, I can only make it once a week. They suggested that they just do a direct transfer in future and I jumped at the chance.
I then called the other relative and apologised, explaining all the same stuff, and asked if the cheques was still ok to cash, or else I was happy to leave it. They said to cash it and would I prefer the cash in future? Again I said yes please!
My birthday was great this year - one of them did a transfer, the other slipped me cash in a card when we met for lunch.
I can understand how your relative cocked up, but if she did realise she had 2 cheques from you then she should have been polite and checked with you whether you had cancelled the first. I think that would look more honest. As it is she either pays very little attention or was chancing her arm a bit.