Nice use case for Layer Actions.

Just configure a new layer action like this:

Code for the layer action:

from qgis.core import QgsProject
from qgis.utils import iface

field_name = "etiqueta"  # CHANGE THIS TO YOUR OWN FIELD NAME

layer = QgsProject.instance().mapLayer("[% @layer_id %]")
idx = layer.fields().indexOf(field_name)

if layer.selectedFeatureCount():
    params = QgsAggregateCalculator.AggregateParameters()
    params.delimiter = ','

    value = layer.aggregate(
        QgsAggregateCalculator.StringConcatenateUnique,
        field_name, params, fids=layer.selectedFeatureIds())[0]
        
    for fid in layer.selectedFeatureIds():
        layer.changeAttributeValue(fid, idx, value)
            
    iface.messageBar().pushSuccess("Concatenate unique", "Done!")
else:
    iface.messageBar().pushInfo("Concatenate unique", "First select some features...")

Notes:

• You just need to change the field_name variable to match your field name and you're done.
• The layer action will be available in editing mode. It won't be saved until you click on Save Layer Edits.

Nice use case for Layer Actions.

Just configure a new layer action like this:

Code for the layer action:

from qgis.core import QgsProject
from qgis.utils import iface

field_name = "etiqueta"  # CHANGE THIS TO YOUR OWN FIELD NAME

layer = QgsProject.instance().mapLayer("[% @layer_id %]")
idx = layer.fields().indexOf(field_name)

if layer.selectedFeatureCount():
    params = QgsAggregateCalculator.AggregateParameters()
    params.delimiter = ','

    value = layer.aggregate(
        QgsAggregateCalculator.StringConcatenateUnique,
        field_name, params, fids=layer.selectedFeatureIds())[0]
        
    for fid in layer.selectedFeatureIds():
        layer.changeAttributeValue(fid, idx, value)
            
    iface.messageBar().pushSuccess("Concatenate unique", "Done!")
else:
    iface.messageBar().pushInfo("Concatenate unique", "First select some features...")

Notes:

• You just need to change the field_name variable to match your field name and you're done.
• The layer action will be available in editing mode. It won't be saved until you click on Save Layer Edits.

Below the code for concatenating several fields at once:

from qgis.core import QgsProject
from qgis.utils import iface

field_names = ["my_field_1", "my_field_2"]  # CHANGE THIS TO YOUR OWN FIELD NAMES

layer = QgsProject.instance().mapLayer("[% @layer_id %]")
field_dict = {layer.fields().indexOf(fn): fn for fn in field_names}

if layer.selectedFeatureCount():
    params = QgsAggregateCalculator.AggregateParameters()
    params.delimiter = ','

    attr_map = dict()
    for idx, field_name in field_dict.items():
        value = layer.aggregate(
            QgsAggregateCalculator.StringConcatenateUnique,
            field_name, params, fids=layer.selectedFeatureIds())[0]
        attr_map[idx] = value
        
    for fid in layer.selectedFeatureIds():
        layer.changeAttributeValues(fid, attr_map)
            
    iface.messageBar().pushSuccess("Concatenate unique", "Done!")
else:
    iface.messageBar().pushInfo("Concatenate unique", "First select some features...")