@eagle_eye : Thanks for the feedback . I too feel such things as time limit between submissions , use of CAPTCHA are all just unnecessary workarounds . I still vouch for separate in contest and end contest test files for CHALLENGE problem .
Hi Vineet, we have made some changes to the scoring of challenge problem starting from October Challenge 2013. You can read them all here: http://blog.codechef.com/2013/10/03/challenge-problem-scoring-changes/