Same Column Names in flextable in R

I am trying to create a 'flexable' object from the R package "flextable". I need to put same column names in more than one columns. When I am putting them in the the "col_key" option in the function "flextable" I am getting the error of "duplicated col_keys". Is there a way to solve this problem?

a<-c(1:8)b<-c(1:8)c<-c(2:9)d<-c(2:9)A<-flextable(A,col_keys=c("a","b","a","b"))

This is the example code for which I am getting the error.