Lexical units with reduplication of word-forming affixes in the texts of the 11-14th centuries reflect the specificity of stylistic features of the Old Russian literature. They also serve as the means of promotion and fixation of important structural and semantic tendencies in the history of Russian. The reduplication of the affixes reflects peculiarity of literary speech, based on the intersection of the Church Slavonic and the original Russian linguistic traditions. It represents different genre-related norms of Old Russian, or norms with variation of expressive means as an inherent feature. Accordingly, the most active models of suffixal reduplication include genetically heterogeneous (Russian and Church Slavonic) synonymic affixes. The contamination of original Russian suffixes with their south Slavonic synonyms is considered to be an instrument of genre and stylistic adaptation of the derivatives to the specifics of a text. The semantic redundancy of words, being the result of the affixal reduplication, corresponds, first of all, to the general stylistic peculiarities of literary speech, and, secondly, is viewed as the basis of semantic development of some lexical and grammatical word classes: in the class of nomina abstracta the reduplication of suffixes contributs to concretization of the meanings of abstract names; in the sphere of verbal prefixation, the "threading" of synonymous affixes became the basis of development of new modifying meanings – the modes of verbal action.