If 1 CCE is enough, due to aggregation, 2 CCE is used to carry DCI.
Data of 2 CCE is same by duplicating or just inserting more redundant bits?
That is why code rate of Agr Lev2 is lower than the Agr Lev1.
So more reliable.
What is the difference between redundant bits and duplication?
Different coding mechanisms
LTE PDCCH uses tail biting convolutional code
NR PDCCH uses Polar code
Duplication or repetition is used for LTE PHICH