Some QGIS calcs

Attributes, features, layers

@layer - current layer
@layed_id - current layer id 

attribute(feature, 'field_name')
attribute(get_feature(@layer, 'fid',  @row_number ),'test')

geom_to_wkt(geometry(get_feature(@layer, 'test', "test")))

geometry(get_feature('test_dst_0bcfae0a_36cb_41c3_b010_3402abf81579', 'test', "test"))

Find dublicated values

count(1,"field-name") > 1