  We investigate the zero-temperature phase diagram and spin-wave properties of
a double exchange magnet with on-site Hubbard repulsion. It is shown that even
within a simple Hartree -- Fock approach this interaction (which is often
omitted in theoretical treatments) leads to qualitatively important effects
which are highly relevant in the context of experimental data for the colossal
magnetoresistance compounds. These include the asymmetry of the doping
dependence of spin stiffness, and the zone-boundary ``softening'' of spin wave
dispersion. Effects of Hubbard repulsion on phase separation are analyzed as
well. We also show that in the ferromagnetic phase, an unusual
temperature-dependent effective electron-electron interaction arises at finite
T. The mean-field scheme, however, does not yield the experimentally observed
density of states depletion near the Fermi level. We speculate that proper
treatment of electron-electron interactions may be necessary for understanding
both this important feature and more generally the physics of colossal
magnetoresistance phenomenon.
