Did the store tell you what gender it was? I don't trust stores much because I was told I had 2 drakes and one turned out to be a duck/hen. Your probably going to have to sex the duck yourself. I found a great video on YouTube that would be every helpful, might be harder to sex an older duck, hope this helps.
Where did you purchase them? (farm/breeder, feed store, hatchery, etc)
Depending on age, you may be able to voice sex them. I don't feel inexperience people should be vent sexing ducklings or adults.
If it quacks, it is a girl. If they sound like they have laryngitis, it is a boy.
They must be around 11 weeks now. I bought them on an agriculture event here and the seller is known for selling chicken breeds but not duck breeds. I wanted a pair of White Pekins just to have around the house since I have a lot of land and no neighbors, but there was only a male so I got him but didn't want him to be alone so I told him I wanted one of the Swedish Blue. When he hands me the duckling his words were: "Here is a female Swedish Blue". So, at least now it was the wrong breed. I'll see if I can identify by their voice. Thank you very much.
I agree it looks like a welsh harlequin but I've also had mixes like that. He could have told you it was a Swedish blue because one of its parents were.I believe the welsh harlequin came from a color mutation in a khaki campbell. Perhaps it's even a mix between a swede and khaki campbell.