Consider

fread("x1,x1,y\n2,3,4", drop = "x1")
#    x1 y
# 1:  3 4
fread("x1,x1,y\n2,3,4", select = "x1")
#    x1
# 1:  2

x1 is duplicated. If both x1 are chosen by select, both should be excluded by drop. At the very least, there should be a warning about multiple x1 detected.

Use case is I have a .csv with a lot of columns called filler which are pointless white space. It would be nice to just say drop = "filler" and exclude all of them.

Originally filed on SO.