  The intent of this paper is to convey a new primary physical idea of a
Dirac-Majorana neutrino duality in relation to the topical problem of neutrino
oscillations. In view of the new atmospheric, solar and the LSND neutrino
oscillation data, the Pontecorvo $\nu - K^0$ oscillation analogy is generalized
to the notion of neutrino duality with substantially different physical meaning
ascribed to the long-baseline and the short-baseline neutrino oscillations. At
the level of CP-invariance, the suggestion of dual neutrino properties defines
the symmetric two-mixing-angle form of the widely discussed four-neutrino
$(2+2)$-mixing scheme, as a result of the lepton charge conservation selection
rule and a minimum of two Dirac neutrino fields. With neutrino duality, the
two-doublet structure of the Majorana neutrino mass spectrum is a vestige of
the two-Dirac-neutrino origin. The fine neutrino mass doublet structure is
natural because it is produced by a lepton charge symmetry violating
perturbation on a zero-approximation system of two twofold mass-degenerate
Dirac neutrino-antineutrino pairs. A set of inferences related to the neutrino
oscillation phenomenology in vacuum is considered.
