John Gill Commentary Genesis 50:17

"So shall ye say unto Joseph, Forgive, I pray thee now, the transgression of thy brethren, and their sin, for that they did unto thee evil. And now, we pray thee, forgive the transgression of the servants of the God of thy father. And Joseph wept when they spake unto him." — Genesis 50:17 (ASV)

So shall you say to Joseph, forgive, I pray you now, the
trespass of your brethren, and their sin
Their very great sin, and therefore more words than one are used to express it: unless this repetition should be intended, and signifies that their crime was a trespass against God, and a sin against their brother; and however they are directed to ask forgiveness for it, and urge the relation they stood in to Joseph, in order to obtain it, which they were ready to acknowledge as a very great evil, and of which they repented:

and now, we pray you, forgive the trespass of the servants of the God
of your father ;
they urge not only the common relation they stood in to Jacob, but what they stood in to the God of Jacob, being his servants, his worshippers, as Joseph also was; and therefore, being his brethren not only in nature but in religion and grace, they hoped he would forgive their trespass:

and Joseph wept when they spoke to him ;
by their messenger; being troubled that they should be in such anxiety and distress of mind, which he had a fellow feeling with, and that they should have no better opinion of him, but entertain such distrust of him, notwithstanding all the kindness he had shown them, as to imagine that he should ever deal hardly with them for their former ill usage of him, which was forgiven and forgotten by him long ago.