The content which is available on multiple URLs on the search engine is duplicate content. Duplicate from the name itself is repeating of two or more exact similar pieces of content from different websites. And since more than one URL displays the same content, search engines might not accept both the webpages and lower their ranks.
Internal Duplicate Content
External Duplicate Content
You can check if your content is duplicate through exact match search, or Copyscape which is a free tool to check if your webpage content duplicates content from other websites. Small SEO tools like plagiarism checkers show the uniqueness and duplication of your content.