Adding field to shp subset layer using PyQGIS?

Question

This is an independent followup from my previous question: QGIS Python generate index field

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I have two files (points.shp, polylines.shp), each with a variable amount of sublayers of the given geometry type.

I need to add a simple incremental index to each of these sublayers: Add a field, and fill it with an increasing, unique integer. Shouldn't be too hard, but I do get more index fields than I desire:

It might be related to the fact that each layer is not an actual individual file, but instead has a source path such as this:

• polylines.shp|layerid=0|subset=Layer = 'pathways'
• polylines.shp|layerid=0|subset=Layer = 'hedgerows'
• points.shp|layerid=0|subset=Layer = 'trees'

For each file:

• The first layer to be processed gets a new index field, and the integer values as intended.
• The second layer gets the index field with the values correctly, but also gets an index_1 field with NULL values.
• The second layer gets the index field correct, and gets index_1 and index_2 with NULL values.
• This continues for every added layer, with the last ending up with dozens of new empty fields (index21 in my case) added.

Why does this happen? How can I prevent it?

This is the processing script:

##input_layers=multiple vector

from qgis.core import *
from PyQt4.QtCore import *

def add_index_to_layer(layer):
    fieldindex = layer.fieldNameIndex("index")
    if fieldindex >= 0:
        print "Layer " + layer.name() + " already has an index field"
        return False
    layer.dataProvider().addAttributes([QgsField("index", QVariant.Int)])
    layer.updateFields()
    layer.startEditing()
    fieldindex = layer.fieldNameIndex("index")
    i = 0
    for feature in layer.getFeatures():
        layer.changeAttributeValue(feature.id(), fieldindex, i)
        i +=1
    layer.commitChanges()

input_layers = input_layers.split(';')
for i in input_layers:
    layer = processing.getObject(i)
    add_index_to_layer(layer)