In this paper we propose and implement a method for detecting duplicate documents in very large image databases. The method is based on a robust "signature" extracted from each document image which is used to index into a table of previously processed documents. The approach has a number of advantages over OCR or other recognition based methods including speed and robustness to imaging distortions. To justify the approach and test the scalability, we have developed a simulator which allows us to change parameters of the system and examine performance for millions of document signatures. A complete system is implemented and tested on a test collection of technical articles and memos.