It depends what your sixthform means by further maths.. For me, I did 12 maths modules, so that's two full A levels in Maths and then Further Maths, but some people just do a few modules and it'll just be an AS.
When people say double maths they mean m and fm being 2 seperate A levels. And not many people would take FM as it's not for people that aren't very good at maths! It's a step up from regular maths, like the first year of maths degree in some respects, so unis respect it as such - or I have found them to have anyway..