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It's definitely a good idea to have out going links that are relevant. I believe it actually helps your SERP. This may help you. https://moz.com/learn/seo/external-link

Hi,

Reading through your question I believe you already know the answer to it. In my opinion I would assume that they will eventually get penalized for duplicate content, your best bet would be to go back to basics of seo and try and differentiate the sites with different content (this may even benefit you), as I'm sure if they are being rented out to your customers they may even appreciate that. Try and make them distinct, and maybe not go down this path again.

For you ranking 15th I believe it is just seo factors and you are going to have to do some trial and error, as the higher you get on rankings, the harder the competition also gets.

I hope this helps and i wish you luck

I'm sorry i cant be a great help here, but something tells me Google prefers just a clean slate singular listing. I probably would not recommend it though as i could imagine it would create some sort of confusion and congestion.

I hope you can find a more detailed answer to your question.

I agree with Highland very much, there is more factors to consider. this may also help you  https://moz.com/beginners-guide-to-link-building

Hi David,

You might find this will help you. https://moz.com/learn/seo/duplicate-content

Other than that, someone else may be able to answer your question in more detail if that helps.