We suggest a way for locating duplicates and plagiarisms in a text collection using an R-measure, which is the normalized sum of the lengths of all suffixes of the text repeated in other documents of the collection. The R-measure can be effectively computed using the suffix array data structure. Additionally, the computation procedure can be improved to locate the sets of duplicate or plagiarised documents. We applied the technique to several standard text collections and found that they contained a significant number of duplicate and plagiarised documents. Another reformulation of the method leads to an algorithm that can be applied to supervised multi-class categorization. We illustrate the approach using the recently available Reuters Corpus Volume 1 (RCV1). The results show that the method outperforms SVM at multi-class categorization, and interestingly, that results correlate strongly with compression-based methods.