You are right in this case, because by the standard notion, a set is a collection of distinct objects, so the union of the members of {{A,B,C},{A,B,C}} is {A,B,C} is indeed meaningless because {{A,B,C},{A,B,C}} is a multiset.
Thank you for your correction.
I offered two examples, neither of which is the one you cited, above.
(1) Union of the members of { {A,B}, {B,C}, {A,C} } is {A, B, C} as follows:
Members of the original set are {A,B}, {B,C}, and {A,C}.
( {A,B} union {B,C} ) union {A,C} = {A,B,C} union {A,C} = {A,B,C}
(2) Union of the members of {A,B,C} is meaningless because the members of the original set, A, B, and C are not themselves sets.
(The not-set nature of A, B, and C was not explicitly stated, only implied in the orginal. Please, let's not waste thread space quibbling over that point.)
By the way is simplifies my claim, which is now based only on the fact that a set is determined by its (distinct) members.
That's fundamental to set theory, that the elements of a set are distinct. Your bogus claim is unchanged.
A set is determined by its members (by its content). Please show me a definition of the container.
Again with this "container" nonsense? A set is a collection of things according to some well-defined rule for membership. Membership requires no containment, it simply is. Why do you feel a container is necessary?
B={{},{{}}}, and { {{},{{}}} } ≠ {{},{{}}}
Was there a point to this last bit?