var pattern = @"  +"; // match two or more spaces
var groups = Regex.Split(input, pattern);

var tokens = groups.Select(group => group.Replace(" ", String.Empty));

var result = String.Join(' ', tokens.ToArray());

// Replace all single whitespaces
for (int i = 0; i < sourceString.Length; i++)
{
    if (sourceString[i] = ' ')
    {
        if (i < sourceString.Length - 1 && sourceString[i+1] != ' ')
          sourceString = sourceString.Delete(i);
    }
}

// Replace multiple whitespaces
while (sourceString.Contains("  ")) // Two spaces here!
  sourceString = sourceString.Replace("  ", " ");

string str = "abc def ghi         xyz";
var result = str.Split(); //This will remove single spaces from the result
StringBuilder sb = new StringBuilder();
bool ifMultipleSpacesFound = false;
for (int i = 0; i < result.Length;i++)
{
    if (!String.IsNullOrWhiteSpace(result[i]))
    {
        sb.Append(result[i]);
        ifMultipleSpacesFound = false;
    }
    else
    {
        if (!ifMultipleSpacesFound)
        {
            ifMultipleSpacesFound = true;
            sb.Append(" ");
        }
    }
}

string output = sb.ToString();

output = "abcdefghi xyz"

public static string RemoveUnwantedSpaces(string text)
{
    var sb = new StringBuilder();
    char lhs = '';
    char mid = '';

    foreach (char rhs in text)
    {
        if (rhs != ' ' || (mid == ' ' && lhs != ' '))
            sb.Append(rhs);

        lhs = mid;
        mid = rhs;
    }

    return sb.ToString().Trim();
}

Regex.Replace("abc def ghi    xyz", "( )( )*([^ ])", "$2$3")

var tmp = Regex.Replace("abc def ghi    xyz", "( )([^ ])", "$2")

var result = Regex.Replace(tmp, "( )+", " ");