How to create a function that eliminates repeated elements

Question

I want to create a function that eliminates repeated elements from a list without changing the order.

For example:

DeleteRepeated[{a, b, 5, a, x + y, a + b, x + y, 7, 5}]

should return:

{a, b, 5, x + y, a + b, 7}

How can I do that in a recursive way? I want to use less then possible the functions done from Mathematica.

PS: It is just for exercise

Answer 1

You can use my very old solution for this:

f[x_] := (f[x]=Sequence[]; x)

DeleteRepeated[list_] := Internal`InheritedBlock[{f},
    f /@ list
]

Then:

DeleteRepeated[{a, b, 5, a, x + y, a + b, x + y, 7, 5}]

{a, b, 5, x + y, a + b, 7}