56 Pharaoh’s double emphasis on “only” uses two different words and was meant to deceive. He was trying to give Moses the impression that he had finally come to his senses, and that he would let the people go. But he had no intention of letting them out.
57 “Death” is a metonymy that names the effect for the cause. If the locusts are left in the land it will be death to everything that grows.

In other versions

Now therefore, forgive my sin, please, only this once, and plead with the Lord your God only to remove this death from me.”

Now forgive my sin once more and pray to the Lord your God to take this deadly plague away from me.”

Please forgive my sin once more and make an appeal to the Lord your God, so that he will just take this death away from me.”

“Forgive my sin, just this once, and plead with the Lord your God to take away this death from me.”

And now forgive my sin surely this time, and pray to Yahweh your God so that he may only remove from me this death.”

Now therefore forgive, I pray thee, my sin only this once, and intreat the Lord your God, that he may take away from me this death only.

Public Domain