After thinking for numerous hours, I'm starting to think that google discovered duplicate content in a unique way.
I have a website that presents photos. I have a desktop version and a mobile version. The differences between the two is that the mobile version is a slimmed down version with fewer features.
This is where I think the problem begins.
On the desktop site, some features have many options, where as on the mobile site, there is only one option. I configure my site so that mobile user who attempts to use an invalid option will be redirected to the correct option.
For example, on a specific gallery of my desktop site, users can choose to view 100, 200, or 500 picture thumbnails per page. In this case, example URLs that could be accessed are:
http://example.com/100-per-page
http://example.com/200-per-page
http://example.com/500-per-page
On the mobile site, users can only view 100 picture thumbnails per page. Example URL is:
http://m.example.com/100-per-page
What I believe google does is attempt to access every desktop link on a mobile device just to see what happens.
I currently have it set so that any desktop URL accessed by a mobile device is redirected to the same URL. The resulting mapping is then as follows:
http://example.com/100-per-page = http://m.example.com/100-per-page
http://example.com/200-per-page = http://m.example.com/100-per-page
http://example.com/500-per-page = http://m.example.com/100-per-page
For every redirect, I issue an HTTP 301 status code. I begin to think that google will think these three URLs are duplicate to each other when accessed from a mobile device:
http://example.com/100-per-page
http://example.com/200-per-page
http://example.com/500-per-page
I did however receive a faulty redirect error on one link when I configured it this way and reading elsewhere makes me thing this error means duplicate content.
I have an idea that I think is ridiculous to implement but I feel its the only way to prevent duplicate content. I feel I need to almost make doorway pages for each URL that isn't available on the mobile site with a message explaining why and what users can do and with using an HTTP 200 status. For example, I feel I could make a page with a text like:
"This feature is only available on desktop computers. On mobile, only 100 thumbnails per page are available. Click the button below to view them"
Is this the way to go or is there a better way to eliminate faulty redirects and duplicate content?