If your resignation has been accepted by CRPF authorities, it would not be possible for you to re-join the earlier post. If your application has been kept pending (sometime the competent authority does this out of sympathy for the employee), you have a chance to re-join the same post after regularisation of the intervening period as leave.
However, as you have resigned, you can again appear for selection against any post of CRPF provided you fulfill the eligibility conditions, and can join CRPF. That would be a fresh appointment.
In this connection, I would like to state that Government recruitment is not a joke, and you can't and shouldn't take appointment in a frivolous manner. Appointment in Government has some responsibility and every post has some dignity. What you did (resigning without thinking about repercussions) is unbecoming of a Government employee. Sorry to say the truth!
Beware! I question everything and everybody.