bool QLineControl::fixup() // this function assumes that validate currently returns != Acceptable
{
...
        if (m_validator->validate(textCopy, cursorCopy) == QValidator::Acceptable) {
            if (textCopy != m_text || cursorCopy != m_cursor)
                internalSetText(textCopy, cursorCopy);
            return true;
        }
...
}

This method is called from FocusOut event handler, which happens when user opens a QComboBox popup, for example. Emitting textChanged signal leads to showing QCompleter popup too. As a result, we see 2 popups.
I suggest simply change line internalSetText(textCopy, cursorCopy); to internalSetText(textCopy, cursorCopy, false); which prevents emiting textEdited signal.