Added check for duplicate column in key
  Added tests and fixed tests which exploit bug


mysql-test/r/delete.result:
  Fix test as it exploited Bug#6252
mysql-test/r/innodb.result:
  Test for Bug#6126
mysql-test/r/key.result:
  Test for Bug#6126/6252
mysql-test/r/type_blob.result:
  Fix test as it exploited Bug#6252
mysql-test/t/delete.test:
  Fix test as it exploited Bug#6252
mysql-test/t/innodb.test:
  Test for Bug#6126
mysql-test/t/key.test:
  Test for Bug#6126/6252
mysql-test/t/type_blob.test:
  Fix test as it exploited Bug#6252
sql/sql_table.cc:
  Bug#6252 - Duplicate columns in keys should fail
    Added check for duplicate column.