I am not 100% sure about sweedish law, however I would assume that customers are not liable for unauthorized transactions on their account. If a transaction was charged twice then they would not be liable for the 2nd charge (if they only authorized the first).
Nordea has promised that customers will get the second charge returned. If the money actually exited their system or if it was just a glitch in their own system is unclear. I am not sure what the law says but I doubt it would come to that as not promising to fix people's balances by reversing the 2nd charge for 2 MILLION people would probably be a bad idea...
What is the biggest problem with this though is that if you went and bought a sofa or something big then you now have a negative bank account balance which may be a problem if you need to buy something such as food for your family.
This is also not a "double spend" in the same way that bitcoin prevents "double spends"
:-) I know. I just wanted to use the phrase since it seemed somewhat appropriate.