string input = "your input string";
StringBuilder sb = new StringBuilder(input.Length);
bool spaceAdded = false;
for (int i=0; i<input.Length; i++)
{
char c = input[i];
if (c == ' ')
{
if (!spaceAdded)
{
sb.Append(' ');
spaceAdded = true;
}
}
else
{
sb.Append(c);
spaceAdded = false;
}
}
string output = sb.ToString();