true if this is the only reference to this list instance; false otherwise.
Complexity: O(n).
auto dl = DList!int(24, 42); assert(dl.isUnique); { auto dl2 = dl; assert(!dl.isUnique); dl2.front = 0; assert(dl.front == 0); } // dl2 goes out of scope assert(dl.isUnique);
Check whether there are no more references to this list instance.