Ifelse statement in R: One value cannot equal another

I have a dataframe with two columns: df$user and df$type. The users are a list of different user names and the type category has two values: 'high_user' and 'small_user'

I want to create some code so that one user cannot be both types. For example if the user = high_user he cannot also be a small_user.

head(df$user)

[1] RompnStomp       Vladiman         Celticdreamer54  Crimea is Russia shrek1978third   annietattooface 

head(df$type)

"high_user" "high_user" "small_user" "high_user" "high_user" "small_user"

Any help would be greatly appreciated.