a merge() problem

I know it does not look very good - using the same column names to mean
different things in different data frames, but here you go:
--8<---------------cut here---------------start------------->8---

x <- data.frame(a=c(1,2,3),b=c(4,5,6))
y <- data.frame(b=c(1,2),a=c("a","b"))
merge(x,y,by.x="a",by.y="b",all.x=TRUE,suffixes=c("","y"))

   a b    a
1 1 4    a
2 2 5    b
3 3 6 <NA>
Warning message:
In merge.data.frame(x, y, by.x = "a", by.y = "b", all.x = TRUE) :
   column name 'a' is duplicated in the result
--8<---------------cut here---------------end--------------->8---
why is the suffixes argument ignored?
I mean, I expected that the second "a" to be "a.y".

(when I omit suffixes, the result is the same).
Thanks.