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 $p_{coll};{sim};10^{-3};-;10^{-4}$ per each resonance scattering, and the line center optical depths for major resonance doublets in the range ${ au}_0;{sim};10^2;-;10^5$. We find that various line ratios are obtained when the product $p_{coll}{ au}_0$ is of order unity. Our Monte Carlo calculations show that the flux ratio can be approximately fitted by a linear function of ${log}{ au}_0$ when ${ au}_0p_{coll};{sim};1$. It is briefly discussed that this corresponds to the range relevant to the emission nebulae of symbiotic stars.