from PySide import QtGui

class TestWidget(QtGui.QLabel):
    def __init__(self, parent=None):
        super(TestWidget, self).__init__(parent)
        self.checkInstances()
        self.setObjectName('myID')
        self.setText('only show me once')

    def checkInstances(self):
        print 'new instance:', self
        for widget in QtGui.QApplication.allWidgets():
            name = widget.objectName()
            if name == 'myID':
                print 'found other instance: %s %s' % (widget, type(widget))
                if self is not widget:
                    print 'closing other instance'
                else:
                    print 'found myself'

nukescripts.registerWidgetAsPanel('TestWidget', 'test widget', 'myID')