library(dplyr)

## 
## Attaching package: 'dplyr'
## 
## The following objects are masked from 'package:stats':
## 
##     filter, lag
## 
## The following objects are masked from 'package:base':
## 
##     intersect, setdiff, setequal, union

left_join(
  data_frame(x = c("a", "a"),
             y = c("B", "C")),
  data_frame(x = c("a", "a"),
             z = c("1", "2")),
  by = "x"
)

## Source: local data frame [4 x 3]
## 
##       x     y     z
##   (chr) (chr) (chr)
## 1     a     B     1
## 2     a     B     2
## 3     a     C     1
## 4     a     C     2