The setting for "Do full justification the way WordPerfect 6.x for Windows does." has nothing to do with copy & paste. Provided anything you copy is justified, if you're pasting the content into the document with that setting applied, it will apply to the pasted content as well. The same goes for plain text pasted into a justified paragraph.

Of course, it's possible the document you're pasting into has acquired some of corruption. Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]