nextpage quicktag, unfortunately, isn’t functional within Page Builder at this time.
Pages or posts built using Page Builder can be cloned from LayoutsClone: Pages or Clone: Posts. It isn’t currently possible to clone Page Builder Layout Blocks built in the Block Editor. If your source page has been built in the Block Editor, you can manually clone it by editing the SiteOrigin Layout block and then clicking LayoutsImport/ExportDownload Layout. In your new page, import the layout by inserting a SiteOrigin Layout block and then click LayoutsImport/Export and import the layout (JSON file).
When duplicating a Block Editor page using the Duplicate Page plugin, you’ll need to manually change the Duplicate Page Editor. This is required because of how Duplicate Page handles editors differently. You can change this setting by navigating to SettingsDuplicate Page and set Choose Editor to Gutenberg Editor.