When ever I attempt to duplicate a selected object I get a warning dialog that says...
An unexpected error has occurred.
Please quit and reopen Keynote.
This error occurs whether I am duplicaing an object on a slide using Command D or duplicating a slide using the right click > Duplicate contextual command.
I am using version 5.3 (1170) of Keynote.
So far this error is not fatal - it is not causing Keynote to crash. Though having to quite and restart means it may as well be. Also I can duplicate without this error using copy and paste - but this is an error with a very high annoyance factor.
I am guessing (sigh) that this will eventually get attention via an update - but until then maybe if enough people click the "I have this question too" link under this post someone at Apple might see it earlier or prioritise the bug.