Duplicate content can be bad for SEO for a number of reasons.

Confusion for Search Engines: When there are multiple versions of the same content on different pages, search engines may have difficulty determining which version is the original and authoritative source. This can lead to a dilution of ranking signals and lower search engine rankings for all pages with duplicate content.

Cannibalization of Keywords: If multiple pages on a website have the same or similar content, they may end up competing for the same keywords. This can lead to a dilution of the SEO value of those keywords and lower search engine rankings for all pages with duplicate content.

Decreased Crawl Efficiency: When search engines crawl a website, they may waste time and resources crawling duplicate pages. This can decrease the crawl efficiency of the website and reduce the overall search engine visibility of the website.

Penalties: In some cases, search engines may penalize websites for having duplicate content. This can result in lower search engine rankings or even removal from search engine results pages.

It is important to ensure that all content on a website is unique and original. This can be achieved by creating high-quality content that provides value to the target audience and using tools such as canonical tags and 301 redirects to consolidate duplicate content into a single authoritative source.