Checks the output of function create_alias_mapping.
check_alias_mapping(Alias_mapping)TRUE if x has the following properties:
Alias_mapping is a non-empty data.frame with character columns 'old' and 'new', without NA and "".
Column 'new' has no duplicates.
Else throws an error. Prints the duplicates of column 'new', if available.
Result of function create_alias_mapping.