I wouldn't use the location of the shape, but give it a specific name (Or, add a 'Tag' property) such as 'Flag'.

Before you add your new shape, loop through the shapes on the slide and if you find one with the proper name or tag, then delete it. And set the name/tag property when you create the new shape.

The Name property is very easy and straigtforward; Using a 'Tag' requires a little more work, but adds a few benefits - you can add multiple tags, the tags are not directly editable by the user, and the tags are a name/value pair, so you could create a Tag named 'Flag' for your shape, and the value could be the type of flag, which could help allow you to identify which flag is used.