roadlist=[]
layer = iface.activeLayer()

for feature in layer.getFeatures():
    if feature['RoadMntnc'] not in roadlist:
        roadlist.append(feature['RoadMntnc'])

for road in roadlist:
    print (road)
    layer.selectByExpression('"RoadMntnc"= road', QgsVectorLayer.SetSelection)
    selection=layer.selectedFeatures()
    for feature in selection:
        print (feature['chainage'])

layer.selectByExpression('"RoadMntnc"= road', QgsVectorLayer.SetSelection)