  Metallicities for six double mode RR Lyrae's (RRd's) in the Large Magellanic
Cloud have been estimated using the DeltaS method. The derived [Fe/H] values
are in the range [Fe/H] = -1.09 to -1.78 (or -0.95 to -1.58, adopting a
different calibration of [Fe/H] vs DeltaS). Two stars in our sample are at the
very metal rich limit of all RRd's for which metal abundance has been
estimated, either by direct measure (for field objects) or on the basis of the
hosting system (for objects in globular clusters or external galaxies). These
metal abundances, coupled with mass determinations from pulsational models and
the Petersen diagram, are used to compare the mass-metallicity distribution of
field and cluster RR Lyrae variables. We find that field and cluster RRd's seem
to follow the same mass-metallicity distribution, within the observational
errors, strengthening the case for uniformity of properties between field and
cluster variables At odds to what is usually assumed, we find no significative
difference in mass for RR Lyrae's in globular clusters of different metallicity
and Oosterhoff types, or there may even be a difference contrary to the
commonly accepted one, depending on the metallicity scale adopted to derive
masses. This ``unusual'' result for the mass-metallicity relation is probably
due, at least in part, to the inclusion of updated opacity tables in the
computation of metal-dependent pulsation models.
