Fix server list error with --long and -c options

Using options --long and -c and specifying same columns added
by --long option, it passes duplicated column names to prettytable and
report the following error:

Field names must be unique!

This patch removes duplicated columns.

Change-Id: I9c0bd09c50dac568ca1980a6b53a6c544b85c2aa

openstackclient/compute/v2/server.py

@@ -2586,9 +2586,9 @@ def take_action(self, parsed_args):
                     columns += ('Metadata',)
                     column_headers += ('Properties',)
 
-            # convert back to tuple
-            column_headers = tuple(column_headers)
-            columns = tuple(columns)
+            # remove duplicates
+            column_headers = tuple(dict.fromkeys(column_headers))
+            columns = tuple(dict.fromkeys(columns))
 
         if parsed_args.marker is not None:
             # Check if both "--marker" and "--deleted" are used.

openstackclient/tests/unit/compute/v2/test_server.py

@@ -4706,6 +4706,7 @@ def test_server_list_column_option(self):
         self.assertIn('Availability Zone', columns)
         self.assertIn('Host', columns)
         self.assertIn('Properties', columns)
+        self.assertCountEqual(columns, set(columns))
 
     def test_server_list_no_name_lookup_option(self):
         self.data = tuple(