If a user already exist with the same user ID then it is not possible to register with the same user ID or with the same email ID and number. There is only one way to recover, and that is from the email ID at the first time when you have created the account you may have received the user ID on your mail, try copy pasting that user ID in the IRCTC website and then try to login. Hope this helps. Otherwise there is no option but to contact IRCTC care or you have to create a new ID with new email and mobile number.
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That’s the irony. I have not received any mail regarding this, that’s why i cannot remember my id. I cant contact customer care helpline as none of the numbers are reachable. Kindly let me know the functional helpline which is in service

Currently you have only one option then, email irctc at care@irctc.co.in and hope you get a reply. One more way is to tweet irctc and ask them to add recover user id by email or mobile number option in IRCTC.