Unnecessary validation messages if labels equal each other

[HEdingfield]

If overvoteLabel, undervoteLabel, or undeclaredWriteInLabel equal one another, unnecessary duplicate error messages are produced by ContestConfig.stringAlreadyInUseElsewhere, e.g. if all three are equal:

2019-07-07 10:02:44 PDT INFO: Validating contest config... 2019-07-07 10:02:44 PDT SEVERE: "overvote" can't be used as overvoteLabel if it's also being used as undervoteLabel! 2019-07-07 10:02:44 PDT SEVERE: "overvote" can't be used as undervoteLabel if it's also being used as overvoteLabel! 2019-07-07 10:02:44 PDT SEVERE: "overvote" can't be used as undeclaredWriteInLabel if it's also being used as overvoteLabel!