IDLE:Make help source menu entries unique and sorted.

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2016-07-08.01:58:45.959>
labels = ['3.8', 'expert-IDLE', 'type-bug', '3.7', '3.9']
title = 'IDLE:Make help source menu entries unique and sorted.'
updated_at = <Date 2019-11-14.13:30:37.183>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2019-11-14.13:30:37.183>
actor = 'taleinat'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2016-07-08.01:58:45.959>
creator = 'terry.reedy'
dependencies = ['27380']
files = []
hgrepos = []
issue_num = 27465
keywords = ['patch']
message_count = 5.0
messages = ['269965', '269966', '316296', '356274', '356606']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'taleinat', 'cheryl.sabella', 'ZackerySpytz']
pr_nums = ['17093']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27465'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']