DouKaku?

xs = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, 8, 9, 7, 9]

h = {}
xs_out = []
xs.each {|i|
  xs_out << i unless h.key?(i)
  h[i] = nil
}

p xs_out