Hash reliability for uniqueness?

Mathematica's associations do not permit duplicate keys. You could make use of this.

I make a simple association to illustrate my point. Since I am only interested in the keys, the values are all zero.

With[{n = 3}, 
  stuff = AssociationThread[Array[thing, n], ConstantArray[0, n]]]

Association[thing[1] -> 0, thing[2] -> 0, thing[3] -> 0]

Appending a new element.

AssociateTo[stuff, thing[42] -> 0]

Association[thing[1] -> 0, thing[2] -> 0, thing[3] -> 0, thing[42] -> 0]

But I can't add a duplicate.

AssociateTo[stuff, thing[2] -> 0]

Association[thing[1] -> 0, thing[3] -> 0, thing[42] -> 0, thing[2] -> 0]

Yes, the order has changed but since it's a hash, the order shouldn't matter.

When the list form is needed:

myList = Keys @ stuff

{thing[1], thing[2], thing[3], thing[42]}

Placeholder answer, because I am just about to be called into a meeting...

I would use Downvalues, and have used them successfully for MMA version 9.0 and earlier

Clear[mylistDownvalues];
mylistDownvalues[_]=False;

(* how you fill with entries, set downvalues *)
mylistDownvalues[entry1]=1;
mylistDownvalues[entry2]=2;
mylistDownvalues[entry3]=3;
mylistDownvalues[entry4]=4;
mylistDownvalues[entry5]=5;

(* trivial to make a helper function *)

myCount=6;
mylistHelper[v_]:=(mylistDownvalues[v]=myCount;myCount+=1)

mylistHelper[entry6];
mylistHelper[entry7];

(* how you get list of entries *)
mylist = SortBy[Select[#[[1,1,1]]->#[[2]]& /@ 
 DownValues[mylistDownvalues],#[[2]]=!=False&],#[[2]]&];
(* Out[]= {entry1->1,entry2->2,entry3->3,entry4->4, *)
(*         entry5->5,entry6->6,entry7->7} *)

(* membership test is fast, insertion is fast, getting index is fast *)

(* membership *)
mylistMemberQ[v_]:=mylistDownvalues[v]=!=False

mylistMemberQ[entry4]
(* Out[]= True *)
mylistMemberQ[entry8]
(* Out[]= False *)

(* getting index *)
mylistDownvalues[entry5]
(* Out[]= 5 *)