If you duplicate an object, you can maximum savings by only moving the duplicate in an orthogonal direction (on a line perpendicular to the original) and not rotating it.
Here's how the PNG compression works: The algorithm catalogues the entire image, finding patterns that are repeated. Those patterns are entered on a hash table, which associates each pattern with a much shorter "key." The key is then inserted into the image file in place of the original pattern. The hash table is then appended to the compressed file.