Whether you create a new site or import an existing web site or web site template, you may be asked for resolving conflicts between existing and imported objects, such as document types with their data tables, web parts, page templates, etc. You're offered with option to choose "winning" object manually or overwrite all existing objects:
If you choose to resolve conflicts manually, you will be guided through all imported objects and you will be able to choose which objects should be imported. If you choose any object to be imported, it will overwrite the existing object. On the following figure, the imported site contains a duplicate definition of CSS style sheet Green that already exists in this system installation. The conflict objects are marked with asterisk (*) on the left. If you check the Import box, the style sheet from the imported site will overwrite the existing CSS style sheet of the same name.
You will continue to other object types until all conflicts are resolved.