  We show that the production of J/psi or Upsilon pairs in unpolarised pp
collisions is currently the best process to measure the momentum-distribution
of linearly-polarised gluons inside unpolarised protons through the study of
azimuthal asymmetries. Not only the short-distance coefficients for such
reactions induce the largest possible cos 4 phi modulations, but analysed data
are already available. Among the various final states previously studied in
unpolarised pp collisions within the TMD factorisation approach, di-J/psi
production exhibits by far the largest asymmetries, up to 50% in the region
studied by the ATLAS and CMS experiments. In addition, we use the very recent
LHCb data at 13 TeV to perform the first fit of the unpolarised
transverse-momentum-dependent gluon distribution.
