Full Idea
With the ordinary view of classes you would say that a class that has only one member was the same as that one member; that will land you in terrible difficulties, because in that case that one member is a member of that class, namely, itself.
Clarification
(the problem is classes that turn out to be members of themselves)
Gist of Idea
Normally a class with only one member is a problem, because the class and the member are identical
Source
Bertrand Russell (The Philosophy of Logical Atomism [1918], §VII)
Book Reference
Russell,Bertrand: 'Russell's Logical Atomism', ed/tr. Pears,David [Fontana 1972], p.126
A Reaction
The problem (I think) is that classes (sets) were defined by Frege as being identical with their members (their extension). With hindsight this may have been a mistake. The question is always 'why is that particular a member of that set?'