Selecting duplicate records in PyQGIS

Question

I have a lot of coordinates in the attribute table like this one:

I'm looking for a way how to find and select active layer duplicated points using PyQGIS. I needn't delete them but only find and show (or select).

Answer 1

This code should do the job (you need to have your layer activated before running it from the Python Console):

layer = iface.activeLayer()

allfeatures={}
index = QgsSpatialIndex()
for ft in layer.getFeatures():
    allfeatures[ft.id()] = ft
    index.insertFeature(ft)

selection = []
for feat in layer.getFeatures():
    inGeom = feat.geometry()
    idsList = index.intersects(inGeom.boundingBox())
    if len(idsList) > 1:
        for id in idsList:
            selection.append(allfeatures[id])
layer.setSelectedFeatures([k.id() for k in selection])

Answer 2

Here is another PyQGIS solution.

Let's assume there is a polygon layer called 'polygon' with its attribute table, see the image below.

Note: Features with the same "id" possess the same geometry.

Proceed with Plugins > Python Console > Show Editor and paste the script below:

# imports
from qgis.core import QgsProject

def select_features_with_duplicates(layer_name: str, reference_field: str) -> None:
    """
    Selects features with duplicates in the reference field
    :param layer_name: name of the layer
    :param reference_field: name of the field to check e.g. id, fid etc.
    """
    
    # get a layer by its name
    layer = QgsProject.instance().mapLayersByName(layer_name)[0]
    
    # list of all values of the reference field
    all_values = layer.aggregate(aggregate=QgsAggregateCalculator.ArrayAggregate, fieldOrExpression=reference_field)[0]

    # make a dict with duplicate values and times each occurred 
    dict_with_duplicates = {value : all_values.count(value) for value in all_values if all_values.count(value) > 1 }
    
    # make a tuple of values from the dict keys
    values_to_select = tuple([*dict_with_duplicates])
    
    # selecting duplicate features by expression
    expression = f'"{reference_field}" in {values_to_select}'
    layer.selectByExpression(expression)
    
    return
    
select_features_with_duplicates('polygon', "id")

Change the parameters of the function in the last line. Press Run script and get the output that will look like this: