Separating Lists by Name

[Tabris]

I have the following scenario

A - 13
A - 12
B - 4
C - 12
A - 2
C - 13

And out of that list I need to create an alternate list for every single letter, where it displays the values already taken. Example

List for A
13
12
2
List for B
4
List for C
12
13

How do I achieve this?

[Bob Phillips]

Wouldn't a pivot table do the job?

[Tabris]

Well not quite.
My final goal is to be able to have a drop-down-list that only displays non-used values of a whole list.
Imagine that I have this list of values under "A" I have to assign to a group of elements by selecting them using a drop down list. Every time a value is selected it shouldn't appear in the list anymore for anyother cell that reffers to it. Preventing the user from selecting the same value twice

EDIT:
To be more specific. Imagine that for the list above there's a drop down list for a new element.
In the first column you choose which letter to use (either A,B or C) and after that the next list should display available values for the given letter (A already used 12, 13 and 2 so those shouldn't appear as a possible choice)

[Bob Phillips]

Take a look at http://www.contextures.com/xlDataVal03.html