And just one more note - it actually tends to be more common in NURBS software to do symmetrical shapes by drawing just half the shape and the mirroring it to produce a 2-segment result.

If you use that method, then be careful how you place the initial 2 and ending 2 control points of the curve. The first 2 points of a curve define the curve's tangent direction so to produce a smooth mirrored results those should be perpendicular to the mirror axis.

- Michael