An example of making ensuring your application is single instance using Mutex. Same can be done using the Process Name, but i found "Mutex" approach much better.
static void Main(string[] args)
{
bool bExists;
Mutex objMutex = new Mutex(false,"TestApp",out bExists);
if(bExists)
{
while(true)
{
Console.WriteLine("Hello");
}
}
else
{
Console.WriteLine("An Instance of application already exists");
Console.ReadLine();
}
}