My two biggest fish of my group of six are female and have always had fights for dominance. The other four are male and have occasional skirmishes with each other.
The fights included locking mouths and pushing each other, circling together and doing it again for around five to ten minutes. At the end of the "fight" the two fish sat on the gravel together as if nothing had happened.
This continued every day until they were twenty years old when one fish, Anthia, got the better of the other fish, Tiama. In the four years since then Anthia has become dominant and Tiama backs down (though she is still No. 2).
I would say my males have a well established pecking order and do not seem to need these fights and a quick skirmish seems to be enough where one fish will "put his foot down". When they were growing up to maturity there was a memorable fight between two equals Mobo and Toby. This lasted several hours, was very vigorous and involved their sub-orbical spines being brandished so it was really serious. However, only exhaustion and some scratches were suffered by the fish and all returned to normal again.
I would say it is normal for these fish to test each other and establish a pecking order and to test again regularly. I would not worry unless it is clearly a stressful situation in which one fish beats up or chases the other all the time.
As said by smee82 it is recommended to keep these fish in shoals. I have been happy with my six in a 110g, but it may be more natural to have many more fish in a shoal in a correspondingly larger aquarium.