Some scholars contend that it has the very same meaning as al-Rahmān; on that reading it would be an emphatic repetition, not a distinct term. Other scholars, however, maintain that its meaning differs, and we cannot simply equate it with al-Rahmān for two reasons:
1 – The default in speech is to introduce a new, foundational idea rather than mere emphasis. Hence, if someone claims that a given word is only reinforcing what precedes it, we reply that, as a rule, each word stands on its own and conveys a meaning other than the first, because emphasis presupposes addition, whereas normal discourse assumes no superfluity.
2 – The two words are built on different morphological patterns: al-Rahmān follows the pattern faʿlān, while al-Rahīm follows faʿīl. A basic principle of Arabic is that a change in form signals a change in meaning.
Therefore, the two names must diverge in sense—but how?
Some scholars say that al-Rahmān denotes the universal mercy, whereas al-Rahīm points to the special mercy. Allah’s mercy is of two kinds:
1 – A general mercy that encompasses all creation.
2 – A special mercy reserved for the believers, as in His saying, “and He is ever Merciful to the believers” (Q 33:43).
Others argue that al-Rahmān refers to the attribute itself, while al-Rahīm refers to the action. In other words, al-Rahmān means “possessor of boundless mercy,” whereas al-Rahīm is the One who causes mercy to reach those who receive it: the first highlights the quality, the second highlights the act.
The view I find most persuasive is the latter: al-Rahmān designates the attribute, and al-Rahīm designates the action.