A subset data frame must include only one level that is meaningful at both levels of the other factor

I have the following df:

Subject Condition Score
 John      Hard     1
 John      Easy     1
 Simon     Hard     2
 Simon     Easy     3
 Mary      Hard     1
 Mary      Hard     2
 Kevin     Hard     2
 Kevin     Easy     3

      

        
        
        
      

    

I would like a subset of this df to only include data from subjects who completed both levels of the "Condition" factor (ie Easy AND Hard). The df result should look like this:

Subject Condition Score
 John      Hard     1
 John      Easy     1
 Simon     Hard     2
 Simon     Easy     3
 Kevin     Hard     2
 Kevin     Easy     3

      

        
        
        
      

    

I've tried the following:

dfNew <- subset(df, Condition==c("Easy", "Hard"), select=Subject)
dfNew2 <- subset(with(df, Subject, Condition == "Easy" & Condition == "Hard"))

      

        
        
        
      

    

But I got error messages and warnings for both.

Any help would be much appreciated.