Hi

I have dual with German and Canadian. First, check with the nearest consulate or embassy for Belgium that is to you. Ask them for the list of paperwork that you will need to prove that you may qualify for a 2nd citizenship.
I had to provide my fathers' birth certificate, when he got his Canadian citizenship, marriage certificate of parents, the long form of my birth certificate and I was lucky in that I also had my fathers' German passport from that time frame as well. Another piece of paperwork that I had that i did not need was when he entered the country.
I hope that this might help you. But if there is no consulate or embassy near you, try doing a google for that and seeing what they have in regards to what their opinions are on having dual. Good Luck.