Navigating to the screen with dropdown twice, gives the following error: Either zero or 2 or more [DropdownMenuItem] were detected with the same value

I know what causes this error, but I couldn't figure it out in this case.
Here is the scenario, when I navigate to the screen that contains the dropdown for the first time, the dropdown works properly. However, when I navigate back and renavigate to the same screen(for the second time) the following error is thrown. Any suggestion?

There should be exactly one item with [DropdownButton]'s value: CategoryDto{ categoryId: cc050c58-876a-4245-9531-be8553c7ae07, name: Masa Oyunları,}. 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value

'package:flutter/src/material/dropdown.dart':
Failed assertion: line 1584 pos 15: 'items == null || items.isEmpty || value == null ||
              items.where((DropdownMenuItem<T> item) {
                return item.value == value;
              }).length == 1'

The following 4 code snippets are in the parent widget where I call the dropwdown. (I want to publish only the relevant parts for convenience.)

This is my parent widget where I call the dropdown.(in parent widget)

  state.categories.isNotEmpty
                                ? CategoryDropdown(
                                selectedCategory: _selectedCategory!,
                                handleSelection: selectCategory,
                                labelText: "Category",
                                categories: state.categories)
                                : Container(),
                          

initialize the doprdown value as null. (in parent widget)
EventCategory? _selectedCategory;

if the API returns the list of categories, then set the first value as _selectedCategory. (in parent widget)

else if (state is CategoryListSuccess) {
                    _selectedCategory ??= state.categories.first;

selectCategory callback function.(in parent widget)

  selectCategory(EventCategory eventCategory) {
    setState(() {
      _selectedCategory = eventCategory;
    });
  }

And this is my CategoryDropdown.

class CategoryDropdown extends StatelessWidget {
  final String labelText;
  final List<EventCategory> categories;
  final EventCategory selectedCategory;
  final Function handleSelection;

  const CategoryDropdown(
      {Key? key,
      required this.labelText,
      required this.categories,
      required this.selectedCategory,
      required this.handleSelection})
      : super(key: key);


  @override
  Widget build(BuildContext context) {
    return Column(
      crossAxisAlignment: CrossAxisAlignment.start,
      children: [
        Padding(
          padding: const EdgeInsets.fromLTRB(6, 0, 0, 0),
          child: Text(
            labelText,
            style: Theme.of(context)
                .textTheme
                .titleMedium
                ?.copyWith(color: Theme.of(context).colorScheme.secondary),
          ),
        ),
        Material(
          elevation: 10,
          borderRadius: BorderRadius.circular(10),
          child: DropdownButtonFormField<EventCategory>(
            value: selectedCategory,
            icon: const Icon(Icons.expand_more),
            elevation: 10,
            onChanged: (EventCategory? category) => handleSelection(category),
            items: categories.map<DropdownMenuItem<EventCategory>>(
                (EventCategory category) {
                  print(selectedCategory);
              return DropdownMenuItem<EventCategory>(
                value: category,
                child: Text(category.name!),
              );
            }).toList(),
          ),
        ),
      ],
    );
  }
}

Thanks to @pskink, I could have solved this. Actually, the problem is not in my flutter code. It's in my rest API, which is written in Java Spring Boot. I generated a dummy category list API.

The following code is wrong. Why? Because each time when I call getCategories from my client(flutter app), it generates a new UUID and sends it to the client. That's why when I first navigate to the screen with dropdown, it makes an API call and selects the first returned element. Then, when I navigate back and, renavigate to the same screen again. I fetch the categories list from the API again. However, since I make a second call this time a new UUID is generated and passed. That's why in the first time the UUID is different from the second time. DropDown throws an assertion error at that point because even though, the name of the two categories are the same, the second one's id is different from the first one. That's why object equality could not be achieved.

    @GetMapping
    public List<Category> getCategories() throws ResourceNotFoundException {
        List<Category> list = new ArrayList<>();
        list.add(new Category(UUID.randomUUID(),"Category1")); //each call creates a new UUID
        list.add(new Category(UUID.randomUUID(),"Category2"));
        list.add(new Category(UUID.randomUUID(),"Category3"));
        return list;
    }

To fix this code I made my list static and pass to the client as follows.

  static  List<Category> categories = fillList();

    @GetMapping
    public List<Category> getCategories() throws ResourceNotFoundException {
        //  throw  new ResourceNotFoundException("Saçamlaamaa");
        return categories;
    }

    private static List<Category> fillList(){
        List<Category> list = new ArrayList<>();

        list.add(new Category(UUID.randomUUID(),"Category1"));
        list.add(new Category(UUID.randomUUID(),"Category2"));
        list.add(new Category(UUID.randomUUID(),"Category3"));
        return list;
    }