When "noindex" is added to a page, does this ensure Google does not count page as part of their analysis of unique vs duplicate content ratio on a website? Yes, that will tell Google that you understand the pages don't belong in the index. They will not penalize your site for duplicate content if you're explicitly telling Google to noindex them.
Is there a chance that even though Google does not index these pages, Google will still see those pages and think "ah, these are duplicate MLS pages, we are going to let those pages drag down value of entire site and lower ranking of even the unique pages". No, there's no chance these will hurt you if they're set to noindex. That is exactly what the noindex tag is for. You're doing what Google wants you to do.
I like to just use "noindex, follow" on those MLS pages, but would it be safer to add pages to robots.txt as well and that should - in theory - increase likelihood Google will not see such MLS pages as duplicate content on my website? You could add them to your robots.txt but that won't increase your likelihood of Google not penalizing you because there is already no worry about being penalized for pages not being indexed.
On another note: I had these MLS pages indexed and 3-4 weeks ago added "noindex, follow". However, still all indexed and no signs Google is noindexing yet.....
Donna's advice is perfect here. Use the Remove URLs tool. Every time I've used the tool, Google has removed the URLs from the index in less than 12-24 hours. I of course made sure to have a noindex tag in place first. Just make sure you enter everything AFTER the TLD (.com, .net, etc) and nothing before it. Example: You'd want to ask Google to remove /mls/listing122 but not example.com/mls/listing122. The ladder will not work properly because Google automatically adds "example.com" to it (they just don't make this very clear).
