from PyQt4 import QtCore, QtGui
import sys
app = QtGui.QApplication(sys.argv)
w = QtGui.QWidget()
w.resize(500, 388)

combo=QtGui.QComboBox(w)
combo.setGeometry(QtCore.QRect(150, 50, 251, 31))
combo.addItem("aa")
combo.addItem("bb")
combo.addItem("cc")
combo.addItem("aa")
combo.setDuplicatesEnabled(False)

w.setWindowTitle("PyQt")
w.show()
sys.exit(app.exec_())