i was going to post exactly the same question as you as i came across it today. I personally think that this is an error because it says...The balance used to weigh the mag carbonate is accurate to 0.01g. Similar questions have appeared in other papers and i used the same method (divide the mass of element and divide it with the balance given e.g 0.01) to answer this one...i would have also done 0.01/ 0.21. It doesn't indicate anywhere that it should be doubled.
In the June 2006 Edexcel 3B, you had calculated the molar mass of a substance as 115 (You did not know what the substance is). http://www.sendspace.com/file/vc5crf (2nd page). Uncertainty in the reading of molar mass for +- 0.91%.
I think that its 0.02 because you have to effectively take two readings - the initial reading without the mass (probably 0.00) and the reading with the mass on it - and both readings have an error of 0.01