Perhaps a better explanation is to look at Mandurah in WA (I have no idea if it is double counted). It is clearly Metro, the southern most area of Perth, and any marketing campaign aimed at Perth has to include Mandurah. However it is also clearly regional, almost as close to Bunbury as the centre of Perth, it gets regional Radio, and TV, and has a regional feel and Outlook. If you are targeting the major regional areas of Perth, you have to include it.
The regional customers are looking for Mandurah figures, but so are Metro customers, what to do?
You put them in both.
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Don't take this personally but the explanation is your best guess on how something you've heard might exist, might work. I agree with your conclusions.
See the link below to ascertain where Mandurah fits.
Back in 2010 Harold Mitchell sold his media buying company to a UK international group for $363 mil - to me that suggests the double counting, as far as it exists is not material and those spending the money are across it.
Of the various markets as counted, Northern NSW is as big a market as Perth and regional Queensland is bigger than Adelaide - the relevance being those who quote the capital city markets as a measure of TV viewing whilst ignoring the larger regional markets are disingenuous or cheerleaders pumping up their own tyres.
http://www.thinktv.com.au/content_common/pg-2016-ratings-toolkits.seo