Would tomatoes and eggs not be sufficient for calcium?
Is she eating the egg shells too?
Egg shells are made of calcium. If she is eating the shells of three or more eggs per day, she might be getting enough calcium (maybe, maybe not). If she is not eating the shells, she is definitely not getting enough calcium, no matter how much she eats of the tomatoes and the insides of the eggs.
Until you get oyster shell, I suggest you offer her the shell of every egg she eats, and every egg that anyone else eats. She may or may not eat them, but offering them at least gives her the option of eating them.
She is one years old and she’s a duck. She usually eats Manna pro duck feed, but she’s been eating tomatoes and eggs as her main meals lately.
Here is a page about what ducks need to eat:
https://grubblyfarms.com/blogs/the-flyer/backyard-ducks-diet
Here is one quote: "Most ducks thrive on a diet of about 1,300 calories per day for adult layers, composed of 16% protein."
You can go add up the calories of what she is eating right now (calories per egg, calories per tomato, how many eggs and tomatoes.)
If she is getting very much less than 1300 calories, give her MORE FOOD. Duck food normally contains quite a bit of grain, and eggs and tomatoes have no grain at all. So I would try adding something that includes grain: maybe bread, or rice, or breakfast cereal, depending on what you have available. (For an idea of how much, read the labels and see.) Most sources recommend not feeding things like bread to ducks-- that may be generally good advice, but in this particular case I think it could be helpful until you are able to get the right duck food again.
Once you have the duck food again, you can adjust how much she gets to try to get her at the right weight. Or you can provide it free choice, as other people have suggested. But for now, you should at least make sure she gets enough food that she doesn't lose any more weight.
Edit to add: I see in one of your other threads that the duck was getting eggs, tomatoes, and corn until someone else told you to cut out the corn. I think you should continue the corn until you can get the proper duck food for her.