Create unique sorted list

Question

I am trying to create a simple function that takes in some lowercase letters a,b,r,e,r etc removes any duplicate letters, and sort them. Then I want to list the output in a list

Input:

\supportMaterials{r,r,e,a}

Output:

\begin{enumerate}
    \item a
    \item e
    \item r
\end{enumerate}

• I tried to use the following code: https://tex.stackexchange.com/a/333666/8306. To create a sorted list, however, no output is given when I try to enumerate the list. Perhaps it expands too late?
• In addition I am not quite sure how to remove duplicates.

Here is my attempt so far

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{enumitem}

\usepackage{expl3,xparse,xstring}

\ExplSyntaxOn
\prg_new_conditional:Nnn \john_string_if_before:nn { p,T,F,TF }
 {% I hope the LaTeX3 police won't catch me
  \int_compare:nTF { \pdftex_strcmp:D { #1 } { #2 } < 0 }
   {
    \prg_return_true:
   }
   {
    \prg_return_false:
   }
 }

\NewDocumentCommand{\sortlist}{smm}
 {
  \IfBooleanTF{#1}
   {
    \clist_set:No \l__john_sortlist_data_clist { #2 }
   }
   {
    \clist_set:Nn \l__john_sortlist_data_clist { #2 }
   }
  \john_sortlist:N \l__john_sortlist_data_clist
  \clist_set_eq:NN #3 \l__john_sortlist_data_clist
 }
\clist_new:N \l__john_sortlist_data_clist

\cs_new_protected:Nn \john_sortlist:N
 {
  \clist_sort:Nn #1
   {
    \john_string_if_before:nnTF { ##1 } { ##2 }
     {
      \sort_return_same:
     }
     {
      \sort_return_swapped:
     }
   }
 }

 \NewDocumentCommand\supportMaterialHelp{m}{%
    \IfStrEqCase{#1}{%
        {r}{R}%
        {b}{B}%
        {k}{K}%
        {c}{C}%
        {a}{A}%
        {e}{E}%
        }[]%
}

 \NewDocumentCommand{\supportMaterial}{ m }
% Input is a list {r,b,k,...} which is defined in the function
% \supportMaterialHelp above. This code loops through the values
% and creates an enumerate with the values
 {%
  \sortlist{#1}{\sortedInputList}%
  \begin{enumerate}[label={--}]%
    % This works fine
    \clist_map_inline:nn { #1 } { \item \supportMaterialHelp{##1} }
    % Why does this fail to work?
    \clist_map_inline:nn { \sortedInputList } { \item \supportMaterialHelp{##1} }
  \end{enumerate}
 }

\ExplSyntaxOff

\begin{document}

\supportMaterial{r,e,k}

\end{document}

Answer 1

If the items are single lowercase letters, you can compare their ASCII codes.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand{\supportMaterials}{m}
 {
  \begin{itemize}
  \nebuch_supportmaterials:n { #1 }
  \end{itemize}
 }

\clist_new:N \l__nebuch_supportmaterials_clist

\cs_new_protected:Nn \nebuch_supportmaterials:n
 {
  \clist_set:Nn \l__nebuch_supportmaterials_clist { #1 }
  \clist_remove_duplicates:N \l__nebuch_supportmaterials_clist
  \clist_sort:Nn \l__nebuch_supportmaterials_clist
   {
    \int_compare:nTF { `##1 > `##2 }
     { \sort_return_swapped: }
     { \sort_return_same: }
   }
  \clist_map_function:NN \l__nebuch_supportmaterials_clist \__nebuch_supportmaterials_print:n
 }

\cs_new_protected:Nn \__nebuch_supportmaterials_print:n
 {
  \item 
  \str_case:nn { #1 }
   {
    {r}{R}
    {b}{B}
    {k}{K}
    {c}{C}
    {a}{A}
    {e}{E}
   }
 }
\ExplSyntaxOff

\begin{document}

\supportMaterials{r,r,e,a}

\end{document}

Answer 2

You do not need any packages for that.

\documentclass{article}
\newif\ifmember
\makeatletter% for \@for see e.g. https://tex.stackexchange.com/a/100684/121799
% from https://tex.stackexchange.com/a/498576/121799
% but with arbitrary stuff instead of integers
\newcommand{\MemberQ}[2]{\global\memberfalse%
    \edef\temp{#2}%
    \@for\next:=#1\do{\ifx\next\temp\relax\global\membertrue\fi}}
\edef\MyItems{a,b,e,r}
\newcommand{\supportMaterials}[1]{%
\begin{itemize}
    \@for\next:=\MyItems\do{\edef\temp{\next}%
    \MemberQ{#1}{\temp}%
    \ifmember \item \temp\fi%
    }%
\end{itemize}}
\makeatother
\begin{document}
\supportMaterials{r,r,e,a}
\end{document}