My website URL contains a #! (hashbang):
www.example.com/index.html
www.example.com/index.html#!
When user visit either two URLs above, the same content appears.
Does Google consider both URLs the same or is there a difference and could, as a result, a negative effect in organic traffic be observed?
Google understands that anchor links are the same page with or without the anchor.
Example: example.com/page and example.com/page#div
Google understands that these are the same pages. And so there is no duplicate content issues.
Anchor links are a very common and useful URL protocol across the web. It can be very helpful for example.com/kittens to link to example.com/animals#cats so that the user can navigate to the related cats content in that div. Linking to your content in this way should be perfectly fine.
Wikipedia links to anchors from different pages. And Wikipedia ranks very highly on Google.
https://meta.wikimedia.org/wiki/Help:Link#Linking_to_anchors
links to:
https://meta.wikimedia.org/wiki/Help:Section#Section_linking
#!throughout your site?#!URLs, this question has info on how Google processes duplicate content and how it could affect SEO: What is duplicate content and how can I avoid being penalized for it on my site?