Wow, 21 weeks and laying! Amazing!
It's perfectly normal for geese to "steal" other egg-shaped objects. But as the duck egg is more fragile, she may trample it.
I agree, it's too late in the season for goslings. Usually the eggs are infertile in Fall, as the gander hasn't done his thing. Also, she may be too young to properly hatch them or care for them.
If you want her to stop laying ASAP, you should leave the eggs in the nest until she starts sitting. But if you want to encourage her to keep laying, just leave one egg (or a golf ball) in the nest. If you keep emptying the nest, she may consider the nest unsafe and choose another location.
Geese usually stop laying and start sitting when they deem they have enough eggs in the nest to sit on. How many that is depends on the goose. A friend of mine has a grown goose who started sitting after laying only three eggs. 8-15 eggs is more likely. But they don't always start sitting this late in the year.
My goose started laying in October last year. I kept taking the eggs, and she produced 108 eggs until June, when she started molting and we decided to let her sit to stop the laying. We filled her nest with golf balls, and marble and porcelain eggs, and within two days she sat. She only left the nest twice a day for half an hour. We let her sit for 9 days, until we were sure the laying had stopped. Then we removed the eggs while she was bathing. When she returned to the nest, she spent 40 minutes searching and then happily abandoned the nest.
BTW, goose eggs taste great and are very good for baking.
