  In this work we investigate the duality linking standard and tachyon scalar
field cosmologies. We determine the transformation between standard and tachyon
scalar fields and between their associated potentials, corresponding to the
same background evolution. We show that, in general, the duality is broken at a
perturbative level, when deviations from a homogeneous and isotropic background
are taken into account. However, we find that for slow-rolling fields the
duality is still preserved at a linear level. We illustrate our results with
specific examples of cosmological relevance, where the correspondence between
scalar and tachyon scalar field models can be calculated explicitly.
