Feature Request

• Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

When limited by max_file_size, sites with a large quantity of "before_posts" data will produce a series of WXR files that could in theory be 100% meta and never reach the point of exporting posts. In the more egregious cases I've seen, 20-50% of each WXR file is taken up with this duplicate metadata.

Before posts:

		$available_sections = [
			'header',
			'site_metadata',
			'authors',
			'categories',
			'tags',
			'nav_menu_terms',
			'custom_taxonomies_terms',
			'rss2_head_action',
		];

Describe the solution you'd like

I would like to work on code to de-deduplicate metadata amongst generated files to save space and processing time and to facilitate the export of extremely large sites. I've filed this as a feature request to see if this use case/new option would be compatible with the export-command.