Hi there Manick,
I don't know whether this answers your question, but it might enlighten you and other readers on possibly why it seems difficult in your case to obtain dual nationality. I believe Winston H. is absolutely right in advising you that dual nationality might be a complicated issue if it relates to a certain point in time. But Cynic is right too, if it concerns a later point in time. Why? Here goes:
If you are in your 50's and left Mauritius when you were 3, then possibly you left Mauritius BEFORE independence (12th March, 1968). Who decided to become a Mauritian citizen, and who decided to stay a British subject was decided by who stayed in Mauritius on the 12th of March and who did not. Everyone who did not want to be a Mauritian citizen left the country before Mauritius became independent. This is the logic behind the nationality law in Mauritius in the very complicated situation of defining who is a citizen of a newly-founded State. The way that it was resolved is that If you were not in Mauritius as from the 12th March 1968 onwards, the newly founded Mauritian State assumes that you decided, or your parent/s or responsible party decided for you that you would remain a British subject.
Those who stayed on in Mauritius could at some point after 1968, obtain dual nationality, and much later on in time, the very rich who are not Mauritian could too.
If you left Mauritius before 12.3.68, you might still be able to obtain Mauritian nationality but that would mean that you would have to legally decline the nationality that you have now, though from your message, it does not seem as if you would want to do that or maybe I am mistaken (?) If you are naturalised British, then I don't know the legal implications in Britain (from what I am reading in international newspapers, nationality of ex-colonised citizens is an issue too in Britain).
But maybe the nationality issue isn't the main issue for you: inheritance law here does state that all movable or immovable property goes to the spouse (if there is one) and children in equal shares. And if you have a birth certificate proving that your parent/s are Mauritian citizens (and as Candycandy has warned you, make sure all the dates and names are correct if you have one) then you would be entitled to inheritance.
Then again, I am not sure what would happen if there is immovable property involved - you cannot own property in Mauritius if you are not a Mauritian citizen unless you have a lot of money in a PDS property scheme.
I suggest you contact a notary versed in Mauritian in citizenship (Mauritian Constitution), inheritance and property law (Mauritian Civil Code).
Best of luck in finding a resolution - I'm sure there must be one. It does help if you understand the why's even if I don't think I've helped you much with the how's. At least you now have a better legal angle to approach the issue.
P.S. - On another note, I have also had issues with wrong entries on my birth certificate and I found that writing to the Ombudsman, Bank of Baroda building, Port Louis was very helpful as well as costing only the postage! The Ombudsman opens an inquiry within 1-2 weeks of receiving a written complaint about maladministration in the pubic sector here, and my issue got resolved.
Just mentioning in case it gets handy at some point....
Cheers,
Malika