Duplicate content in landing pages or anywhere else will badly affect your website's google search engine ranking. If because of technical reasons, duplicate content is being created then you can find some best practices mentioned over here. But for intentionally created duplicate content will not work good for search engine ranking.

I think that soon SE will consider that as the same page. So only one of them will be in the index and second one will be simple ignored or something like that.

Yes, Adding duplicate content in the website is harmful to the website and SEO. First, content is the reason a website get traffic from the Internet but if you will add duplicate content in your website, the users wont use your website. And second GOOGLE rewards the unique contents. The duplicate contents slow down the webmaster because the webmaster become confused with duplicate content and that is why there is some penalty for duplicate contents.

You will not be able to rank above your competitors with duplicate contents.

You have to accept the Google Law that whenever the word "Duplicate" comes that means you are in trouble so please try to use original things

It may possibly affect the ranking as SE's like Google and Bing always like unique and quality content. In your case, both the landing pages will be indicating to the same root domain. Hence if the content remains the same, it will certainly somehow affect the website SE Rank.

duplicate content will always hurt. you can not save your self by hurting if you have rough or duplicate content on your site.

It will hurt search rankings, you should use .htaccess to redirect one of them to the other or or noindex in one of those

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See duplicate content can bring DMCA complaints but since its your on property so you will not face that
Only issue I can see is that Search engine would get confuse which Url to rank for and that can be a problem for you. Instead of doing this there's a thing called A/B testing you could have followed that like sending X amount of traffic to page A and Y to page B and see which is giving more success to you.

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Yes duplicate content will affect your seo efforts.

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Does duplicate content work the same way with eCommerce products? The same product many times across multiple websites, with same specifications?

Regards

Google will know content was created before or after. If you use all duplicate content in landing page, it will effect your website rank in search engine.

It will not matter as both the pages have same content. It is better to redirect one to the other and use it. You can find Google's policy for redirecting a page at :- support.google.com/webmasters/answer/93633?hl=en