In this review, the effects stemming from double passage of reflected and backscattered waves through the same inhomogeneities of the medium, are discussed. The enhancement of phase and intensity fluctuations and of the average intensity and other effects, characteristic of double passage, are described in detail. Multichannel coherent effects, arising due to mutual coherence of different waves passing in opposite directions along identical channels, are investigated. The experimental work on double passage effects is reviewed. Possible practical applications of these effects are analyzed.