Delete repeating list elements preserving order of appearance

Question

I am producing flat lists with 10^6 to 10^7 Real numbers, and some of them are repeating.

I need to delete the repeating instances, keeping the first occurrence o

Answer 1

For versions of Mathematica before 7, and for general interest, here are several ways of implementing the UnsortedUnion (i.e. DeleteDuplicates) function. These are collected from the help docs and MathGroup. They have been adjusted to accept multiple lists which are then joined, in analogy to Union.

For Mathematica 4 or earlier

UnsortedUnion = Module[{f}, f[y_] := (f[y] = Sequence[]; y); f /@ Join@##] &

---

For Mathematica 5

UnsortedUnion[x__List] := Reap[Sow[1, Join@x], _, # &][[2]]

---

For Mathematica 6

UnsortedUnion[x__List] := Tally[Join@x][[All, 1]]

---

From Leonid Shifrin for Mathematica 3+ (?)

unsortedUnion[x_List] := Extract[x, Sort[Union[x] /. Dispatch[MapIndexed[Rule, x]]]]