merge two itunes libraries without creating duplicates windows 10 itunes?

If you want to merge two libraries then you are going to lose the date added details from one of them. Typically you would merge the smaller library into the larger. In this case connect to the smaller library and use File > Library > Export Library... to export the library as an XML file. Connect to the other library and use File > Library > Import Playlist... to import the XML file. See this post to eliminate any Duplicate Songs. Consolidate the library if it isn't already. Then see Backup your iTunes for Windows library with SyncToy to make sure you have a backup.

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