According to wikipedia chickens cannot lay more than one egg in 24 hours. I'm sure it isn't good for them, especially as their life-span is closely related to the number of ovum they use up, so laying too many double-yolked eggs will shorten their life, or so I have been told by someone who ought to know.
I'm sure it is neither normal, nor desirable, but Robin has proved it is possible for a chicken to lay more than one egg in 24 hours. I noticed she was in the nest box last night, although hubby had collected 3 eggs in the morning. One of those eggs had a very thin shell, though, and it was cracked. The egg I collected about twenty past seven last night was smooth and pale, but the shell was thick enough.
This morning I collected two eggs from the nest box, presumably from Amber and Betsy. Later on I noticed Robin pop up into the nest box, but she came down again and nothing was there. This happened again. Then about quarter past twelve I realised I'd forgotten to replace the roosting bar after cleaning the floor of the chickens' sleeping quarters. When I went to do that I noticed Robin was missing and could hear her in the nest box again. At ten to one I found a smallish, pale and very warm egg in the next box. Unfortunately this one also had a cracked shell as it was so thin. Poor Robin, I hope she sorts herself out soon.
Update, 20th May 2009. We have been getting a very regular supply of three eggs a day from our three pullets for the last couple of weeks, so it seems this was just a teenage blip. To be honest Robin seemed to just start laying with no problems from the word go, so she was due a short spell of irregularity, I suppose.