  Color-kinematics duality is a remarkable conjectured property of gauge theory
which, together with double copy, is at the heart of a wealth of new
developments in scattering amplitudes. So far, its validity has been verified
in most cases only empirically, lacking a first-principle understanding beyond
tree-level. In this paper we provide initial steps in a first-principle
understanding of color-kinematics duality and double-copy at loop level,
through a detailed analysis of the field-theory limit of the monodromy
relations of string theory at one loop. In this limit, we dissect the type of
Feynman graphs generated and the relations they obey. We find that graphs with
contact-terms are unavoidable and are generated in the field theory limit of
"bulk" contours which do not have a standard physical interpretation in string
perturbation theory. We show how they are related to ambiguities in the
definition of the loop momentum and that their role is precisely to cancel
those ambiguities.
