The simplest way is just a nested loop:
public static string GetRepeatedSequential(int start, int end, int repeat)
{
StringBuilder sb = new StringBuilder();
for(int i = start; i <= end; i++)
{
for (int j = 0; j < repeat; j++ )
{
sb.AppendLine($"{i:D4}");
}
}
return sb.ToString();
}
To do it using Linq methods like Repeat isn't that complicated either, but does look clumsy:
public static string GetRepeatedSequential(int start, int end, int repeat)
{
return string.Join(Environment.NewLine,
Enumerable.Range(start, end - start + 1)
.SelectMany(i => Enumerable.Repeat($"{i:D4}", repeat)));
}