I don't know about ducks, I expect someone else will tell you about them, but it's quite common in pullets just 'learning' to lay their first eggs - what is actually two egg yolks get somehow coated with shell and turned into one egg. What surprises me, Paddy, is that you've had three double-yolkers out of 8 when she's been laying every day. I thought that usually a double-yolk egg is basically two day's worth, that is you get her missing a day and then producing a double egg. Obviously you've been looking after her so lovingly that she's really trying her best to please you! I'm glad to hear that you've managed to enjoy some of her eggs, though, so her efforts aren't being wasted!