R: Generate a dummy variable based on the existence of one column' value in another column

I have a data frame like this:

A                    B          
2012,2013,2014     2011
2012,2013,2014     2012
2012,2013,2014     2013
2012,2013,2014     2014
2012,2013,2014     2015

I wanted to create a dummy variable, which indicates whether the value in column B exists in column A. 1 indicates the existence, and 0 indicates non-existant. Such that,

A                    B       dummy        
2012,2013,2014     2011        0
2012,2013,2014     2012        1
2012,2013,2014     2013        1
2012,2013,2014     2014        1
2012,2013,2014     2015        0

I have tried to use %in% to achieve this:

df$dummy <- ifelse(df$B %in% df$A, 1, 0)

but it turned out that everything in the column of dummy is 1.

Same situation happened when I tried to use another method any():

df$dummy <- any(df$A==df$B)

everything in the column of dummy is TRUE.

Is there an efficient way to generate this dummy variable?

Many thanks!