Copying, pasting and undo has inconsistant behaviour in QTextEdit

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.