Can a leader also be a follower?

Yes, without a doubt. Every leader can be a follower of another leader and that would actually make that leader even better if he follows someone who he think he can learn and educate himself from.

This type of a leader, who is also a follower, is a very good form of leadership. If you do not have any mentors, coaches or anyone else to look up to you have a good chance of "slipping off the vagon" and actually turning into a bad leader in the long run even if you are doing quite well now.

In theory, a single person can be a leader for multiple communities or organizations as well as be also a followers for many other leaders.