The MakeUnique(L) function removes all repeated elements from the list L.

The elements are reviewed in order from left to right. If an element matches a previous element, it is removed from the list.

The order of the elements in the returned list matches the order in the list L.

The MakeUnique(L, N) function retains N copies of matching elements in list L. If greater than N matching elements exist in L, they are removed.

By default, the elements in the list are compared by using boolean comparison. If argument f is specified, then f(x, y, opt1, opt2, ...) is called to check if element x and element y should be considered equal. This function should implement an equivalence relation (i.e. it should be reflexive, commutative, and transitive).  If it is not, then the result may not be valid.

$\mathrm{with}\left(\mathrm{ListTools}\right)\:$

$L≔\left[1\,2\,3\,4\,3\,2\,3\,4\,5\,4\,3\,4\,5\,6\right]$

$L≔\left[1\,2\,3\,4\,3\,2\,3\,4\,5\,4\,3\,4\,5\,6\right]$

$\mathrm{MakeUnique}\left(L\right)$

$\left[1\,2\,3\,4\,5\,6\right]$

$\mathrm{MakeUnique}\left(L\,0\right)$

$\mathrm{MakeUnique}\left(L\,2\right)$

$\left[1\,2\,3\,4\,3\,2\,4\,5\,5\,6\right]$

$\mathrm{MakeUnique}\left(L\,'\mathrm{\infty }'\right)$

$\left[1\,2\,3\,4\,3\,2\,3\,4\,5\,4\,3\,4\,5\,6\right]$

$L≔\left[0.\,\left(x-1\right)\left(x+1\right)\,-0.\,x^2-1\,0\,-\left(1-x\right)\left(x+1\right)\right]$

$L≔\left[0.\,\left(x-1\right)\left(x+1\right)\,\mathrm{-0.}\,x^2-1\,0\,-\left(1-x\right)\left(x+1\right)\right]$

$\mathrm{MakeUnique}\left(L\right)$

$\left[0.\,\left(x-1\right)\left(x+1\right)\,\mathrm{-0.}\,x^2-1\,0\,-\left(1-x\right)\left(x+1\right)\right]$

$\mathrm{MakeUnique}\left(L\,1\,\mathrm{verify}\,\mathrm{expand}\right)$

$\left[0.\,\left(x-1\right)\left(x+1\right)\right]$

ListTools:-MakeUnique([1,2,6,5,10,9,7,3,11],
                      2,
                      proc(a,b)
                          evalb(a-b mod 4 = 0);
                      end proc
                     );

$\left[1\,2\,6\,5\,7\,3\right]$