Restart autoincremental numbering by unique values

You could create a custom script which increments the value index you created and saves these in a new field. You can create one from:

Processing Toolbox > Scripts > Tools > Create new script

Then use something like the following:

##Example=name
##Layer=vector
##Field_to_group_increment=field Layer
##New_index_field=string index
##Output=output vector

from qgis.core import QgsField, QgsVectorFileWriter, edit, QgsVectorLayer
from PyQt4.QtCore import QVariant

layer = processing.getObject(Layer)
writer = QgsVectorFileWriter.writeAsVectorFormat(layer, Output, "utf-8", None, "ESRI Shapefile")
output = QgsVectorLayer(Output, 'any_name', 'ogr')

idx = output.fieldNameIndex(Field_to_group_increment)
values = output.uniqueValues(idx)

with edit(output):
    output.addAttribute(QgsField(New_index_field, QVariant.Int))
    for feat in values:
        x = 10
        for feature in output.getFeatures():
            if feat == feature[Field_to_group_increment]:
                feature[New_index_field] = x
                output.updateFeature(feature)
                x = x + 1

del writer

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Make sure to save the script into your /.qgis2/processing/scripts directory. You can then run the script from the Processing Toolbox or include it in your model.

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Example:

The interface when you run the script:

The result: