Parsing FlagCounter argument fails with `isDefault` Verb

Parsing FlagCounter arguments fail, when used together with default Verb feature with following error
```
ERROR(S):
Option 'v' is defined multiple times.
```
code sample:

```
[Verb("option", isDefault: true)]
class Options
{
[Option('v', FlagCounter = true)]
public int Verbose { get; set; }
}

class Program
{
static int Main(string[] args)
{
args = "-vv".Split(' ');
var parser = new Parser(opts => { opts.GetoptMode = true; });
var parsed = parser.ParseArguments(args);

Console.WriteLine(parsed.Value == null ? "Parsing failed" : "Flag parsed");
return 0;
}
}

```

When either specifying Verb explicitly (`args = "option -vv"`), or when counter value is <= 1 (`args = "-v"`) or when not using Verbs parsing `parser.ParseArguments(args)`, the sample above completes and successfully parses the flag value.

從提供的 C# 重現開始，將使用預設 verb 和 FlagCounter 的 ParseArguments 與明確 verb、單一 flag 和非 verb 情況進行比較。完成標準是預設 verb 情況能夠成功解析 "-vv"，且不會回報 Option 'v' 被定義多次。

由索引模型根據 Issue 內容生成。