As long as you have exact match content on 2 different URL's - without using precautions to block indexing, or canonical tag implemented, you will have duplicate content. Here is an example that might give you the answer you are looking for, since I can't really give you a more direct answer without looking at how the blog software you are using, is publishing the page in terms of multiple tags.
If you publish a blog-entry with 2 categories, to www.site.com/blog/post-A/ but the same page is accessible on the URL: www.site.com/blog/category/category1/post-A/ and www.site.com/blog/category/category2/post-A/ - without use of the above mentioned ways of preventing duplicate content, yes you could be suffering from duplicate content.
Most popular blogging tools has measures to prevent this in the administration back-end or as add-on modules..
