unique / sort in data.frame

I have a data frame like this:

x=c("01","01","01","22","22","03","03","03","35","35")
y=c("f","f","f","m","m","m","m","m","f","f")
df=data.frame(spn=x, sex=y)

seems like:

   spn sex
1   01   f
2   01   f
3   01   f
4   22   m
5   22   m
6   03   m
7   03   m
8   03   m
9   35   f
10  35   f

What I'd like to do is to sort the df$spn and let it appears only once. the appropriate df$sex as well, like:

   spn sex
1  01   f
2  03   m
3  22   m
4  35   f

How could I do that? many many thanks!