In the Arabic languange, there are two forms of plural: one of them is the broken plural, and the other is rafa' plural according to a combination. For example, in the Qur'an, both kuffarun and kafiroona are used as the plural form of kafir (disbeliever). Similar examples can be found throughout the Qur'an and the hadith literature, and the Arabic language in general.
What is the difference between these two plural forms? Why is there a need to have two sets of plurals?