What is the reason behind dpkg including files in several packages despite being mentioned in only one package's .install file?

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The reason for this could be that the file is shared by multiple packages and hence needs to be included in each of those packages. This is often seen in libraries or common files that are used by several applications. Including the file in each package ensures that the dependencies are satisfied for each package, and the application functions properly. Additionally, this allows users who only need one application to install it without having to install the entire collection of packages.