Short Answer:
This is a general message, and PocketEarth will not download the same data twice.
Long Answer:
PocketEarth is smart enough to *never* download the same data twice. In particular, if you download cities or regions and then download the containing region or country it will not download the data that it already has. Similarly neighboring regions or countries which share some common data near the border will not download that common data twice.
The situation with the place download dialog and the size reported in it is more complicated:
• For Regions and Countries, the size reported is the actual size the place will take on your device, but some of that data may be re-used by other places. Also, some of the reported data size may have been already downloaded, thus the actual download size may be much smaller.
• For Cities, the size reported is only an estimate of the download size, and the actual download size may be more or less, but usually the difference is less than 1 MB.
We plan on improving this situation with the reported size in a coming update, as well as offering more control over what data is downloaded.