Show message when alias already exists in /alias add

tests/test_cmd_alias.h

@@ -3,6 +3,7 @@ void cmd_alias_add_shows_usage_when_no_value(void **state);
 void cmd_alias_remove_shows_usage_when_no_args(void **state);
 void cmd_alias_show_usage_when_invalid_subcmd(void **state);
 void cmd_alias_add_adds_alias(void **state);
+void cmd_alias_add_shows_message_when_exists(void **state);
 void cmd_alias_remove_removes_alias(void **state);
 void cmd_alias_remove_shows_message_when_no_alias(void **state);
 void cmd_alias_list_shows_all_aliases(void **state);