Have just updated Matrix to 2.5.6.
When I go to Add-ons > Fieldtypes > Matrix to trigger the update script I get this error.
A Database Error Occurred Error Number: 1060
Duplicate column name 'var_id'
ALTER TABLE exp_matrix_cols ADD var_id INT(6) UNSIGNED AFTER field_id, ADD INDEX (var_id)
Filename: third_party/matrix/ft.matrix.php
Line Number: 496