This is purely informational. I think very few people will be affected by this bug. If you're seeing replication of the content of "Card" Notes fields though, it might help to know about it. I don't know of a workaround yet, but I'm still studing the problem.
I discovered it when I migrated from MobileMe to iCloud two days ago and found that several hundred of my Contacts/Cards had extra line feeds (blank lines) in the Notes field text, and that with each synchronization the content of the Notes field was being replicated.
I believe this is the cause (excerpted from a longer blog post) 
When I inspected my iCloud "Cards" I found that not every Card/Contact Note field had problems. The ones with problems not only showed duplication of Notes content, they also showed extra spaces between paragraphs. The problems were also restricted to Contacts/Cards that started life in Microsoft Outlook, and that moved via MobileMe to OS X.
This smells like a a new line definition aka line termination aka line break aka end-of-line (EOL) marker bug. One of the most intractable and maddening bugs in the history of software (it's been a curse, for example, for Blogger.)
Windows uses CR+LF to define an 'end of line' - or paragraph break. Mac Classic used CR. OS X uses LF. My Address Book contains contacts that started out in Outlook, then went via MobileMe Windows Control Panel to live in MobileMe, and then into my desktop machine.
MobileMe could handle the CR vs CR/LF variation. So can Address Book in Lion and in Snow Leopard. It appears, however, that iCloud.com cannot handle a mixed set of Contacts with variable EOL markers.
To make matters worse, because iCloud.com and OS X are managing the EOL encoding differently, each time my iCloud contacts sync with a new machine the Notes field is treated as non-matching, so the contents are appended. This repeats until a Notes field maximal length limit is hit.
It's a nasty bug. I'll be experimenting with ways to work around it, such as exporting an Address Book archive, then reimporting as UTF-8 or Windows or Mac rather than "automatic". Perhaps Address Book will do the conversion for me.
Ironically, I came across this line feed distinction when I pasted the above text in from a web page source. I had to add line feeds to define paragraphs.
The CR/LF vs. LF/CR vs CR vs LF schism in software history gets my vote for worst schism ever.