  We investigated experimentally and theoretically dielectronic recombination
(DR) populating doubly excited configurations $3l3l'$ (LMM) in Fe XVII, the
strongest channel for soft X-ray line formation in this ubiquitous species. We
used two different electron beam ion traps and two complementary measurement
schemes for preparing the Fe XVII samples and evaluating their purity,
observing negligible contamination effects. This allowed us to diagnose the
electron density in both EBITs. We compared our experimental resonant energies
and strengths with those of previous independent work at a storage ring as well
as those of configuration interaction, multiconfiguration Dirac-Fock
calculations, and many-body perturbation theory. This last approach showed
outstanding predictive power in the comparison with the combined independent
experimental results. From these we also inferred DR rate coefficients,
unveiling discrepancies from those compiled in the OPEN-ADAS and AtomDB
databases.
