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The existence of an impurity-activated Eg resonance mode in NaCl:Cu+ has been suggested by several previous experiments. Raman data presented here reveal this resonance directly and also reveal the three components of the first overtone of the 23.5-cm-1 infrared resonance mode. The frequencies of the Eg resonance and the Eg component of the overtone are shifted as a result of a strong anharmonic coupling. Their line shapes and strengths are considerably altered by an interference between the Raman amplitudes. A reasonable fit to the data has been obtained using a simple theory.