A friend has done this to me...i've had a boys name picked since before I was married and I live abroad so I thought an English name would be safe and original. It never crossed my mind she'd even like the name since she isn't english so when we were discussing genders, I said it would be great to have a boy as I've had his name chosen for 8 years.
Well she is 2 months ahead of me and found out shes carrying a boy and announced she's calling him by the name I had picked out. I thought it was a joke at first and asked her if she was serious....she said 'absolutely, and I've bought personalized items with his name on'. Like you I was sooooo annoyed, I felt extremely betrayed. When I initially found out I said to DH we'll have to think of another name but he point blank refused.
So I've taken a new approach - I've made sure that everyone that knows me and her knows the names I've chosen - I tell people that I know couldn't care less but I'm making sure they know where they heard it first (I never mention that she's also using the name)....also, theres an addition detail, I've also told them that the boys name we've chosen is my grandfathers name (which isn't strictly true between me and you) but now no one will question who chose that name....so you could could make a big facebook announcement that you are calling your son Rueben after DH grandfather/long long cousin/uncle etc. I know it's a white lie but after a week if feeling so deflated, I decided not to get mad - to get even!