presto-jdbc/src/main/java/io/prestosql/jdbc/WarningsManager.java/WarningsManager/addWarnings
class WarningsManager:    public synchronized void addWarnings(List<Warning> warnings)
    {
        for (Warning warning : warnings) {
            if (warningsSeen.add(warning.getWarningCode())) {
                addWarning(warning);
            }
        }
    }