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It is not uncommon to import an object that already exists in your repository; after all, you are just replacing one definition with another but it is less likely that you're replacing say, a Field with a Process, or an Icon with a Form, so to prevent you from accidently deleting object definitions, the import will not automatically proceed if it would mean the same identifier will end up being used for a different type of object.
If it's an intentional change however, you can select this option and proceed with the import.  In this case the existing object will be deleted and the new object definition will be imported.
 
 
⇑ 4. Editor Features