“In Genesis 6:19, 20 God tells Noah to place in the ark two of every sort of animal. In Genesis 7:2, 3 the Lord tells him to take seven each of the clean animals and two of the unclean. Is this a contradiction or two writers’ differing points of view?”
“And of every living thing of all flesh, you shall bring two of every kind into the ark, to keep them alive with you; they shall be male and female. Of the birds after their kind, and of the animals after their kind, of every creeping thing of the ground after its kind, two of every kind will come to you to keep them alive” (Genesis 6: 19, 20 New American Standard Bible).
God gave these initial instructions to Noah when He first commanded him to build the ark. The purpose of the instructions here is to prepare Noah for the work ahead, which was to build a boat large enough not only to sustain him and his family, but also the animals of the world. God intended to repopulate the earth with pairs of animals rescued from the flood by Noah obedience.
“You shall take with you of every clean animal by sevens, a male and his female; and of the animals that are not clean two, a male and his female; also of the birds of the sky, by sevens, male and female, to keep offspring alive on the face of all the earth” (Genesis 7:2, 3).
This last, more detailed set of instructions is given to Noah after he has finished the ark and is about to enter into it (7:1). For many years Noah labored on this great project. At the completion, God doesn’t change the instructions, He amplifies them. Just as God told Moses He would go with him to Pharaoh and do wonders, only later did God amplify and show Moses all the wonders he were to do in front of Pharaoh. In other words, God started Noah off with the overall plan and then gave him more details as the plan progresses. It is not a contradiction nor does it indicate more than one author.