presto-main/src/main/java/io/prestosql/sql/analyzer/StatementAnalyzer.java/StatementAnalyzer/validateColumnAliases
class StatementAnalyzer:        private void validateColumnAliases(List<Identifier> columnAliases, int sourceColumnSize)
        {
            validateColumnAliasesCount(columnAliases, sourceColumnSize);
            Set<String> names = new HashSet<>();
            for (Identifier identifier : columnAliases) {
                if (names.contains(identifier.getValue().toLowerCase(ENGLISH))) {
                    throw semanticException(DUPLICATE_COLUMN_NAME, identifier, "Column name '%s' specified more than once", identifier.getValue());
                }
                names.add(identifier.getValue().toLowerCase(ENGLISH));
            }
        }