Two nests, one duck?

My girlfriend and I are guardians of a château in Normandy that has a pond and about fifteen domestic ducks (in addition to a couple wild Mallard couples).
The domestic breeds include three White Muscovies, one of which started laying eggs about four weeks ago. The last time I was able to make a count of the eggs, about two weeks ago, there were thirteen in the nest. A few days ago I noticed there were three eggs outside of the nest and figured she must have rejected them or something.
Today, however, I went to check on the nest and I discovered not one but two nests with at least twenty two eggs in total (ten in the new one).
Now, it's quite possible that she was continually laying eggs after I was able to keep count, her first nest is deep and well lined with down. So maybe she felt it had filled up, then started laying eggs outside of it, and then finally decided to make another nest and move a few of the older eggs over with the new ones? Is this at all plausible? If so, would that mean these are not fertile eggs, and she is simply going to continue producing eggs, and we should start eating these things?
Another possibility is that one of the other ducks has started to lay eggs in the duck house. The eggs do look a bit different but perhaps that is just because they are younger? I imagine it would have to be the case that I overlooked a couple eggs the other day because from what I understand ducks only lay an egg a day and I only saw three about three days ago, so there should be six not ten if it is another's nest. Though my overlooking the eggs is, of course, also a possibility.
Also, it is far from clear that the other two Muscovies are male. As far as we know they could be but we haven't seen any mating going on, while with the other duck breeds have been going wild lately.
Thanks so much in advance for any help answering these questions.

I think that there is a good chance that more than one female duck is laying in the nest. When we had two breeds of domestic ducks who laid different colored eggs, we would often eggs from more than one duck in one spot.

You did not say weather one of the ducks was sitting on the eggs to incubate them and hatch them. She would be on the nest all the time except to eat and take a quick swim if she was trying to hatch them. Domestic ducks, like chickens will lay eggs without getting the urge to sit on them. The eggs that were laid four weeks ago will start to rot and smell soon (unless she has been incubating them, which means they will hatch soon.) From you description, I would guess that the female ducks have been laying eggs with no intention of hatching them. That means it is probably time to clean out the nest. Once you have cleared out all the old eggs, you would have the option of eating any more eggs that they laid if you wanted to collect them on a regular basis.

The female has been very diligent with her sitting. She is on her eggs all day. We see her for 15-20 minutes in the afternoon, when she comes to get some food.
However, I still don't know for sure that the eggs are fertile. Apparently, Muscovy eggs take about 35 days to hatch, which means they should be hatching late this weekend. If not, well I suppose it may be time to pull the eggs. Is there any obvious signs at this point that an eggs is fertilized?
As far as the other nest goes. There is no down in the nest yet and the color of the eggs is still quite different, so I think another hen has been putting her eggs in the duck house. She doesn't seem particularly interested in sitting, however. Should I start pulling the eggs if she doesn't sit soon? It's really pretty cold outside.. freezing at night.
Thanks!
Also here is a photo

You can't tell if they are fertile by just looking at them in the nest. I think the hen may have pushed out the ones that she thought were not good. You might find some ducklings under her in a few days.
As far as the other nest, I think that a duck would start sitting on the eggs after about a week. The eggs also may have frozen over night if they were not covered.

Hi All! Please share your knowledge! We have a nest and originally we had seen 2 blue eggs there, the mommy was coming and going. As of today, it's a 3rd day we do not see mommy duck, but we have a party of 5 male ducks (drakes) swimming in the pool. I know for good one of 5 is a daddy duck, he swims back and forth and stretching his head to check on the nest, but no mommy! Is it possible that duck can have 2 nests the same time, or she had abandoned the nest in our yard?!