Where from the second half-wave of rectified voltage comes in full-wave rectification?
A full-wave rectifier is a specific arrangement of diodes.  It has nothing to do with windings, frequency doubling, etc.

The diodes are arranged so that whichever of the two AC wires is most positive, this positive voltage passes through to to the same rectified output node. I've marked up a diagram to show the current paths. Please forgive the crudeness of my artwork!



As you can see, when the "upper" AC node is positive, it drives current through the load from top to bottom.  When the the "lower" AC node is positive (which is the same as saying that the "upper" node is negative), the current is still driven through the load from top to bottom.  The load never sees a negative voltage.

Does this help?