Understanding how nofollow works and what it's meant for
Your biggest mistake is your misunderstanding of how nofollow works - a lot of people assume that nofollow means Google will not follow the links, this is not true. Nofollow is considered a attribute that is best used for off-site links, quote from Johns recent answer on another related question:
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Generally speaking, nofollow should be used on the following type of
links:
Untrusted content
Paid links
Tackling the duplicate content problem
With the birth of Blog engines such as WordPress created a headache for webmasters a like with duplicate pages such as Tag, Author, Recent, Date, Popular pages etc, Google confronted this issue many years back with the release of rel="canonical", by using canonical pages you are telling search engines that this is the master page and all other pages that the content is found on is ignored without punishment for duplicate content.
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If Google knows that these pages have the same content, we may index
only one version for our search results. Our algorithms select the
page we think best answers the user's query. Now, however, users can
specify a canonical page to search engines by adding a element
with the attribute rel="canonical" to the section of the
non-canonical version of the page. Adding this link and attribute lets
site owners identify sets of identical content and suggest to Google:
"Of all these pages with identical content, this page is the most
useful. Please prioritize it in search results."
Yoast SEO and many alike
Adding canonical pages to WordPress is a walk in the park thanks to many plugin authors such as Yoast SEO, by using such plugins you can easily add REL Canonical to your master pages and make the duplicate content problem go away for good. I've used Yoast as my example as I've used this plugin for many sites and it works well but there are many more plugins that are equally as good which you can explore on the WP plugin library.
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When Google introduced the canonical link element, to distinguish the
“original” page from derivative pages within your site carrying the
same content, they reached out to me to develop a WordPress plugin for
it, and I did. Later on, canonical link elements were added to core.
They work fine, with one caveat: they only work for single posts and
pages, not for categories and tags, not for the homepage. The
WordPress SEO plugin fixes that, and sets the correct canonical on
each of those pages.
XML Sitemap
The priority settings is should not be considered a reliable option to conquering duplicate content issues, you should opt to use Rel Canonical instead and leave the sitemap untouched.
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Q: If I have multiple URLs that point to the same content, can I use
my Sitemap to indicate my preferred URL for that content? A: Yes.
While we can't guarantee that our algorithms will display that
particular URL in search results, it's still helpful for you to
indicate your preference by including that URL in your Sitemap. We
take this into consideration, along with other signals, when deciding
which URL to display in search results.
