When placing a block into a region via the core block UI onto a page that is a non-default Context Layout (using an alternate page.tpl.php) blocks are shown twice. By placing the same block via context the block is shown just once.

My justification for mix-and-match using Context and core...
I personally use context when multiple blocks will be on a page, or a block will be on multiple pages.
But the core block UI seems appropriate with a single block needs to be on a single page.