presto-spi/src/main/java/io/prestosql/spi/type/TimeZoneKey.java/TimeZoneKey/put
class TimeZoneKey:                @Override
                public synchronized Object put(Object key, Object value)
                {
                    Object existingEntry = super.put(key, value);
                    if (existingEntry != null) {
                        throw new AssertionError("Zone file has duplicate entries for " + key);
                    }
                    return null;
                }