Finding values in one column with several different values in another column in R : I have a large data set with a column for the identity and a birthdate. Unfortunately, some id names were used for different individuals. 


`sample = data.frame("id" = c("val1", "val1", "val1", "val1", "val2", "val2", "val2", "val3", "val3", "val3", "val3"),
                    "birthday" = c("1", "1", "1", "1", "2", "3", "4", "5", "6", "7", "7"))`

Now I'm trying to find which id's have different birthdates to be able to rename them. 

I know I could do it with a for loop but I was wondering whether there is an easier & faster way? Or how would you deal with it?

Thank you very much

Nina
