edge case with duplicate cols when by is different on two sides

[moodymudskipper]

x <- data.frame(a=1,b=1)
y <- data.frame(a=1,c=1)
eat(x,y, .by=c(a="c"), .check = "C")
#   a b a.y
# 1 1 1   1

This shouldn't work, and it works silently, not safe. We apply our standard conflict function.

[moodymudskipper]

done