So i have one male and one female duck. They are both back yard pets and the female has recently started laying one egg a day! Is it normal for them to only lay one egg at a time and does it mean they arent fertile?

No your duck is fertile and perfectly normal as laying one egg a day is all that they're capable of doing.

You didn't say what breed, or where you live. Here in Fla. We get a lot of different breeds. The one thing in common is being very close to water, and having buildings or heavy brush to lay eggs next to. If she is laying eggs in open areas, it's a lost cause, she claims no worth to them - unless she is trying to entice the male. Normally she will find a spot, dig a bit of a hole, in the shade of a building or heavy brush, that won't be deluged with rain. And use her beak to pull dead grass,weeds and such closer, to create a nest. If your in the south, she will leave unattended because of enviromental warmth. If in the North, She will only leave occasionally. Janey is correct, that most females lay one egg a day. I suggest you google your breed and area, to know better what's going on, and what you can expect. Good Luck. Quacky Quack!