As other people say it’s hormones. But I do believe there is something, not much, you can do to discourage it.
Take away the fake egg.
Take away laid eggs asap.
Never let her sleep in a nest box.
A fresh and cool roost area:
Make very very much ventilation in the roost area. Like 2 sides with hwc on the sides where no storms can be expected. At least from spring till autumn.
Con: sometimes I find eggs on strange places. Like under the roost, on top of the compost heap, behind a garbage container… the uncoloured but very real easter eggs.