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This block is somewhere in your design package layout files.
Try to find “compare” keyword using search function in your editor in following files (paths might be little different becasue i’m writing from memory):
App/design/frontend/[your package]/default/layout/sales.xml or
Or you can enable template path hints in backend. You can find how-to in google
I think the question is not, how to edit compare, its about a general thing. So main question is, if a 1col, 2col-left/-right or 3 columns template is wished. What you want to show in sidebar? Content? CMS Blocks? Images? Nothing?