merge data frames with same column names of different lengths and missing values

x <- data.frame(item1=c(NA,NA,3,4,5), item2=c(1,NA,NA,4,5), id=1:5)
y <- data.frame(item1=c(NA,2,NA,4,5,6), item2=c(NA,NA,3,4,5,NA), id=1:6)

merge(x,y,by=c("id","item1","item2"),all.x=T,all.y=T) #my rows are duplicated

retained - I instead want one row per ID
  id item1 item2
1  1    NA     1
2  1    NA    NA
3  2     2    NA
4  2    NA    NA
5  3     3    NA
6  3    NA     3
7  4     4     4
8  5     5     5
9  6     6    NA