There should be exactly one item with Dropdownbutton's value – Flutter

Im very begginer in flutter and I’m trying to put 4 dropdownbuttons and when I change the value for the second one I get error:

There should be exactly one item with [DropdownButton]’s value: Dollars.
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
‘package:dropdown_button2/dropdown_button2.dart’:
package:dropdown_button2/dropdown_button2.dart:1
Failed assertion: line 1128 pos 11: ‘items == null ||
items.isEmpty ||
value == null ||
items.where((DropdownMenuItem item) {
return item.value == value;
}).length ==
1’

There is the first one working:

String lang = 'English';
 var items = [
   'English',
   'Polish',
 ];
DropdownButtonHideUnderline(
             child: DropdownButton2(
               items: items
                   .map(
                     (item) => DropdownMenuItem<String>(
                       value: item,
                       child: Text(
                         item,
                         style: const TextStyle(
                           fontSize: 14,
                         ),
                       ),
                     ),
                   )
                   .toList(),
               value: lang,
               onChanged: (String? newValue) {
                 setState(() {
                   lang = newValue!;
                 });
               },
               
             ),
           ),

And the second one that isn’t

String currency = 'Dollars';
  var curriencies = [
    'Dollar',
    'Euro',
    'PLN',
    'Funts',
  ];
DropdownButtonHideUnderline(
                    child: DropdownButtonFormField2(
                      items: curriencies
                          .map(
                            (currency) => DropdownMenuItem<String>(
                              value: currency,
                              child: Text(
                                currency,
                                style: const TextStyle(
                                  fontSize: 14,
                                ),
                              ),
                            ),
                          )
                          .toList(),
                      value: currency,
                      onChanged: (String? newValue) {
                        setState(() {
                          currency = newValue!;
                        });
                      },

I changed the values, but it’s still same