How to intantiate random prefab each time ,that is different from another

Game that is im going to make looks like from Google s dinosaur game.
But instead of dinosaur and obstacles,i have player and alphabet letters.

Each letter is being spawned every time,that is moving towards player, the aim is colliding with a letter that is in strict alphabetic order. For example, each time you begin as ‘A’ letter, and next letter (which is random)that is moving towards you must be ‘B’,if it is so collide with a letter, if not jump over it ,otherwise game restarts.
In this way you must get all letters in alphabetical order, say a,b,c,d…
My problem is ,say ,I begin with A ,and if B letter coming I must collide with it.after Collision A becomes B.
The problem is when A becomes B ,no other B must be spAwned.but since game is not in slow pace after collision,one more B is being spawned, even though its random,and not always it happens, but it happens!
So how to fix this issue?

You need to keep a list of what you HAVE spawned and then the next spawn go through the list, and compare if it is there. If so repeat the spawn process again. Alternatively keep a list of letter you WANT to come next, and select one of those random, then remove it from the wanted list. That way you can save some time if the list of used letters is getting big.