If I received a severance package, that was put in a RRSP. Why do I to pay taxes on that amount, then pay taxes again when I use it?

Received the severance in February of 2017. Put all of the severance in an RRSP, and claimed that in 2016. In 2016 there was no  box 67. I took the deduction. Now they have said I have to claim it as income and owe then a lot of money. I want to know if they are right, and why am I going to be taxed twice. Why put anything in to an RRSP.