Short answer is, no. There are exceptions, of course, but the number of champs that are worth building multiples of is pretty limited. From my own collection, the only two that I've built multiple of are Coldheart and Royal Guard. I'll also be building a second Taurus when i get it, but only because of the Elenaril OTK combo.

No, it's not. It's even debatable if you need both - it's useful as it gives you more possibilities for affinity CB, but on void, once your team is fully built, you generally only need one of the two, both together is poison overkill. 

Oh I thought Frozen Banshee was mainly there for the Poison Sensitivity, to support another poisoner. Is one better than the other? If I can get rid of one of them, then who would I want to bring in their place?

@L9753 @Quintinar Thanks guys, I'll try some different teams and see how it goes. Good to know I can get rid of the duplicates at least.

It depends on how your team is built - and she needs to be booked to properly function (OB doesn't). If she's fast enough and has the right masteries, she can do plenty of poison damage just by herself - ideally with a secondary poisoner who also does other things in support, but not a hardcore poisoner like OB. If you have a counterattack team, with a champion like Skullcrusher, she'll do a lot more poison still since she'll often counterattack with her A1. 

Which one being the best also depends on your team in general, but roughly you can say they're more or less equally good. As I mentioned, affinity matchups will also play a role - FB is particularly bad against her weak affinity (force) because if you get a weak hit on the poison sensitivity, she'll do almost nothing for the next three rounds until you can try again. So in that sense having both options is definitely good, unless you're always hitting void CB anyway (but ideally that isn't the case - you should be in an alliance that manages to kill the CB on your target difficulty, so you get double reward chests every day). 

Frozen Banshee is absolutely amazing.  You don't need her and another dedicated poisoner being on the team at the same time (although an Aox or Fahrakin the fat type of "poisoner" is ideal).  Pretty much everything is contingent on your team though.  If you are running a slow team it is different than if you are runnig a faster team or a counterattack team or an ally attack team.  As L9753 mentioned though, having both is a good idea because affinity matters a lot.

It all depends on what else you have in your collection and what your plans are for using two Frozen Banshees or Occult Brawlers.

Not sure if I agree with that statement. If you run FB without a CA or AA champ, you don't really get much value out of her. Unless you gear her to go 2:1, her kit essentially amounts to putting two poisons every three turns, which is definitely not great. If you have other poisoners, you get much more value out of her A3, and if you run two FB, you can keep poison sensitivity permanently up, which gives you far more value out of both than the sum of their parts.

Put another way - a single FB on a 1:1 rotation gives you two poisons every three turns (discounting the scenarios where you extend to 3 turns for poison sensitivity). Two FB on 1:1 rotation, on the other hand, gives you eight poisons every three turns, IE, an average of 33% more poison stacks per FB.

Short answer is No. Long answer is No.

Building  2  Frozen  Banshee  to  max  isn't completely out  of  the  question.

I  know  a  few  players who  have  built  up  2  Frozen  Banshee.

I  have  also  thought about  it  myself  at  times.

The  reason  a  person considers  building  2x  FB  isn't necessarily for both  of  them  in CB.

The  reason  they  build  up  2  is  to  have  1x  FB dedicated to  CB  +  1x  FB  dedicated to  something else.

For  Example:

I  use  FB  for  Dragon  Stage  24,  Nether  Spider  Floor  90,  &  UNM  Clan  Boss

Against  the  Dragon  &  Nether  Spider  -  I  want  my  FB  to  be  tanky  because  they  hit  hard.

Against  the  CB  -  I  want  my  FB  to  be  glass  cannon  because I  run  Block  Damage  team.

My  FB  gear  builds  for  CB  are  different vs.  Dragon  &  Nether  Spider  builds.

This  is  why  I  have  thought about  building up  a  2nd  FB.

Two FB won't do anything.

One FB uses up to 5 debuff slots. You want 1 slot for atk down, 1 slot for def down, 1 slot for leech, 1-2slots for weaken/hp burn.

It is recommended to take the debuff extender mastery for FB, that gives her sometimes 3 rounds sensitivity.

A second poisoner is too much wether you run counterattack or not.

I am running FB in relentless and the debuff slots get ssometimes overfilled. Ninja and Sepulcher as the two other debuffers

One FB cannot use 5 debuff slots without CA/AA. Turn 1 - poison sensitivity. Turn 2 - two poisons. Turn 3 - poison sensitivity is gone, so no more poisons.

Relentless is a really bad set for CB. You throw your rotations completely off. Highly recommend you drop it.

T1 sensitivity, mastery kicks in extended to 3 turns...

T1 sensitivity, Aox extends it.

T1 sensitivty in a countereattack setup and banshee fills the bar by herself.

The account I am using FB/relentlsess can 2key brutal but is worlds away from NM. Don't have the champs and gear for Nightmare. Worlds away from the need to speed tune anything.

The other account uses Aox, Banshee, Valk, Sandlashed, Ninja. Banshee in crap gear and way too slow (140 speed) and still the debuff bar gets overfilled. 4key NM with Ninja at 50...Mostly in crap gear. And gear matters more than champs, yes right.

I have 2 built frozen banshees although i use only one nowadays

1 is fast, and i used to run on dragon

the second is my slowboi for a budget unkillable (clan boss)

NO