All clubs play one another twice - and play three other clubs from a fixture pool and additional set of home and away fixtures.
Depending on the results of previous season’s standings clubs are put into four fixture pools who play one another home and away. This gives 30 + 6 = 36 games each.
In North American sports it is perfectly normal for the more successful clubs to play one another more often.
The additional points up for grabs for the additional games between the clubs in the same fixture pool would be more balanced leading to more competition.
Stronger teams playing weaker teams only twice increases the chance of upsets due to unfamiliarity.
If the SPL and SFL 1 ended today then the following fixture pools would be created to decide fixtures for next season:-
1/ Celtic, Rangers, Hearts, Kilmarnock
2/ Inverness, Motherwell, Dundee Utd, St Johnstone
3/ Aberdeen, Hibs, St Mirren, Hamilton
4/ Dunfermline, Dundee, Raith , Falkirk
In other words, Celtic, Rangers, Hearts and Kilmarnock would play one another four times per season and play the other 12 clubs only twice.
See this thread for discussion:
Is this a better proposal for 16 team SPL with 36 fixtures?