duplicate ads

Good evening. I created 3 blocks on the home page in different locations. Each block contains two different advertisements which must run between them. So basic are Block 1 (A-B), Block 2 (C-D), Block 3 (E-F). What I would like to achieve is that the ads mix without creating duplicates, for example Block 1 (F-A), Block 2 (C-E), Block 3 (A-B). The problem that every page load has is creating duplicate advertisements, sometimes even tripled. How can I do? Can you give me advice? Thank you.