Add checking the duplication entry to exclude_parser.py

More than one entry in Problem_list will cause all other former entries be invalid. JTREG will only take the last one as valid entry. This includes not only exactly same entry but also same testcase entry with different tracked issue or platforms.

Start in exclude_parser.py and trace how Problem_list entries are parsed and passed to JTREG. Determine how duplicate entries currently cause earlier entries to become invalid, including identical entries and the same testcase with different tracked issues or platforms. Done means the requested duplication checking is present for these cases.

Written by the indexing model from the issue text.