1)
Why does it say "Ketz Ba Ba ha'Ketz"?
1.
Radak: The repetition is to strengthen the matter and hasten it.
2.
Malbim: This is unlike other evils, which come to fix the nation - they will repent and endure. This is an end and eradication.
2)
Why does it add "Hekitz Elayich"?
1.
Radak: This is Hif'il conjugation. Its root is Katz - the middle letter of the root is sometimes omitted. The root of Ketz is Katzatz.
2.
Malbim: The verse depicts that it is a day special for evil - a full day, including the night. The night already passed - the Ketz and eradication slept, for night is a time for sleeping. Now the Ketz Hekitz (woke) and began to do its action.
