idle_test: test_fontlist_key() fails if two font families have the same name

[vstinner]

BPO	39600
Nosy	@terryjreedy, @vstinner, @taleinat, @serhiy-storchaka, @csabella, @miss-islington
PRs	bpo-39600, IDLE: Remove duplicated font names #18430 [3.7] bpo-39600, IDLE: Remove duplicated font names (GH-18430) #18436 [3.8] bpo-39600, IDLE: Remove duplicated font names (GH-18430) #18437 [3.8] bpo-39600, IDLE: Remove duplicated font names (GH-18430) #18441 [3.7] bpo-39600, IDLE: Remove duplicated font names (GH-18430) #18442 bpo-39600: Adjust code, add idlelib/NEWS item #18449 [3.8] bpo-39600: Adjust code, add idlelib/NEWS item (GH-18449) #18450 [3.7] bpo-39600: Adjust code, add idlelib/NEWS item (GH-18449) #18451

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = <Date 2020-02-11.04:24:24.636>
created_at = <Date 2020-02-10.15:44:03.184>
labels = ['3.8', 'expert-IDLE', '3.7', 'tests', '3.9']
title = 'idle_test: test_fontlist_key() fails if two font families have the same name'
updated_at = <Date 2020-02-11.11:59:49.184>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2020-02-11.11:59:49.184>
actor = 'vstinner'
assignee = 'terry.reedy'
closed = True
closed_date = <Date 2020-02-11.04:24:24.636>
closer = 'terry.reedy'
components = ['IDLE', 'Tests']
creation = <Date 2020-02-10.15:44:03.184>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 39600
keywords = ['patch']
message_count = 15.0
messages = ['361700', '361701', '361712', '361721', '361727', '361728', '361729', '361730', '361767', '361769', '361770', '361771', '361772', '361773', '361802']
nosy_count = 6.0
nosy_names = ['terry.reedy', 'vstinner', 'taleinat', 'serhiy.storchaka', 'cheryl.sabella', 'miss-islington']
pr_nums = ['18430', '18436', '18437', '18441', '18442', '18449', '18450', '18451']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39600'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[vstinner]

======================================================================
FAIL: test_fontlist_key (idlelib.idle_test.test_configdialog.FontPageTest)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/usr/lib64/python3.7/idlelib/idle_test/test_configdialog.py", line 104, in test_fontlist_key
    self.assertNotEqual(down_font, font)
AssertionError: 'Cantarell' == 'Cantarell'

Example:
---

import tkinter.ttk
import tkinter.font
frame = tkinter.ttk.Frame()
families=sorted(tkinter.font.families(frame))
for family in families:
    print(family)

---

Truncated output on my Fedora 31:
---
Abyssinica SIL
Android Emoji
AnjaliOldLipi
Bitstream Vera Sans
C059
Caladea
Cantarell
Cantarell
Cantarell
Cantarell
(...)
Comfortaa
Comfortaa
(...))
DejaVu Sans
DejaVu Sans
DejaVu Sans
(...)
Source Han Serif CN
Source Han Serif CN
Source Han Serif CN
Source Han Serif CN
Source Han Serif CN
Source Han Serif CN
Source Han Serif TW
Source Han Serif TW
Source Han Serif TW
Source Han Serif TW
Source Han Serif TW
Source Han Serif TW
(...)
---

I'm not sure if it's an issue in the unit test or an issue with the widget itself. Does it make sense to display the same font family name twice?