You can mention that the hard copy of the appointment letter will be provided only on submission of the relieving letter. Until then the employee can have a soft copy (this must not have company letter head logo/design).
Or let him show proof of resignation acceptance letter.
If you feel something fishy abt this new employee, pls ask him for references from his previous company, and you can call the HR of the previous company by ursefl. say that you are an outside agency and u r just doin the background verification... in this way u need to reveal the companys' name where this new employee has joined. And also you will get to know the truth - if the person has really resigned or is playing to fool everyone.
From India, Madras
The new one can produce the resignation acceptance letter can work out in this situation. Candidate can forward his conversation (Mails) on his resignation and acceptance to the new employer. With out Relieving and Resignation Acceptance letter we should not entertain the new joiner.
From India, Bangalore