A syndrome is defined as a group of symptoms that occur together. If one person has "classic CFS" and one person has "lookalike CFS" and there's no diagnostic d

A syndrome is defined as a group of symptoms that occur together. If one person has "classic CFS" and one person has "lookalike CFS" and there's no diagnostic differentiator (eg they both exhibit the same symptoms and no test will tell them apart) then by definition they both have the same syndrome. If sounds like you're defining CFS as a distinct illness which it is not, that's why it's called a syndrome.