An-Nisa: 122
"And those who believe and do righteous deeds..."
"The promise of Allah is true."
Two verbal nouns (maṣdarān): the first serves to emphasize itself, and the second serves to emphasize something other than itself.
"And who is more truthful than Allah in statement?"
A third, eloquent emphasis.
If you ask: What is the benefit of these emphases?
I say: It is to contrast the false promises of Satan and his vain desires for his companions with the truthful promise of Allah for His allies. This is to encourage the servants to prefer that by which they deserve the fulfillment of Allah’s promise, over the bitterness they will swallow in the end from the breaking of Satan’s promises.
An-Nisa: 123 - 124