When you "Export as Catalog" you choose a location and name for this new Catalog. Whether relevant files are to be copied here too, or not, is controlled by a checkmark option "Export negative files".
IF checked, a set of subfolders is created alongside the new Catalog, corresponding to the relative arrangement of subfolders seen in the current catalog - and populated with copies of the image files concerned. The new Catalog references these copy files and copy folders. The current Catalog continues to reference all the originals as before.
So the files have been duplicated - with the copies separated out systematically, but the originals still intermixed. Thus, cleaning these images out from the current Catalog - using the same highlighted set that you used for the Export - can include a deletion from disk as well as a removal from the Catalog, and then there is no more duplication.
OR if not checked, the new catalog references (the same) files in the same current folders where the current catalog continues to see them - in other words, both the catalogs share these same files and folders. So in this case one's deletion of the relevant images from the current catalog needs to use the Remove from Catalog option only. For, I hope, obvious reasons.
[FYI: when exporting (say) contents of a single Collection, all of which have been highlighted, the first checkbox may not appear. But when it does, if this is left unchecked, all of the images in the catalog will get included regardless of highlighting or not. So these options are important to verify.]
Also - whenever considering removing images from any Catalog, I would advise ensuring an up to date Catalog backup first. The Export as Catalog process is itself non-destructive so far as the current Catalog and the current locations of all the image files - whether copying files or not - but the cleanup of the current Catalog afterwards does need to involve a lot more care.
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