Resonance doublets including O VI 1032, 1038, NV 1239, 1243 and C IV 1548, 1551 constitute prominent emission lines in symbiotic stars and planetary nebulae. Spectroscopic studies of symbiotic stars and planetary nebulae from UV space telescopes show various line ratios of these doublets deviating from the theoretical ratio of 2:1. Using a Monte Carlo technique, we investigate the collisional de-excitation effect in these emission nebulae. We consider an emission nebula around the hot component of a symbiotic star characterized by the collisional de-excitation probability Pcoll~10-3-10-4 per each resonance scattering, and the line center optical depths for major resonance doublets in the range T0~102-105 We find that various line ratios are obtained when the product PcollT0 is of order unity. Our Monte Carlo calculations show that the flux ratio can be approximately fitted by a linear function of logT0 when T0Pcoll~1 It is briefly discussed that this corresponds to the range relevant to the emission nebulae of symbiotic stars.