Copying, pasting and undo paragraphs has inconsistant behaviour in QTextEdit when the actions are repeated more than once in the same document.

Can be reproduced like this:
1. Run demos/textedit and create new document.
2. Type two paragraphs like:
aaa
bbb
3. Copy these paragraphs.
4. Paste them at the end of second paragraph and press Enter.
5. Now we have four paragraphs like:
aaa
bbbaaa
bbb
(empty paragraph)
6. Press undo (1), undo (2).
7. Remove everything and repeat the actions in the same document.