“Let them be put to shame and dishonor who seek after my life! Let them be turned back and disappointed who devise evil against me! Let them be like chaff before the wind, with the angel of the Lord driving them away! Let their way be dark and slippery, with the angel of the Lord pursuing them!” -Psalm 35:4–6
There are four “Lets” in this pericope all petitioning the Lord for a certain outcome against the enemies of David.
For those who seek his life, he prays for their shame and dishonor, for it is a shameful and dishonorable thing to seek the life of a righteous man.
For those who devise evil against him, he prays for their plans to be disappointed and for the wicked to be turned back because evil devices are themself a privation of that which is good and therefore cannot achieve goodness.
For these wicked men who plot against him, David prays they will be blown away like chaff because they are themselves empty, worthless men. Further, he prays their way will dangerous and hindered with darkness because their motives are dark.
And, in both the latter cases, he prays the angel of the Lord himself will drive these men away and pursue them like a predator pursues its prey or a guard dog chases away a thief in the night.
To say virtually the same thing in a different way repeatedly is merely a poetic way of emphasizing the greatness of his need and the deficiency of his own ability to defend himself from the evil men who want to destroy his life.