[BUG] file.accumulated won't accumulate duplicate text

Description

When using file.accumulated, duplicate text fields can not be added to a given named accumulator.

Setup

brothers.sls:

accumulate_distinct_brothers:
  file.accumulated:
    - name: uncledonald
    - filename: /tmp/brothers.conf
    - text:
      - Huey
      - Dewey
      - Louie
    - require_in:
      - file: brothers.conf

accumulate_duplicate_brothers:
  file.accumulated:
    - name: newhart
    - filename: /tmp/brothers.conf
    - text:
      - Larry
      - Darryl
      - Darryl
    - require_in:
      - file: brothers.conf

brothers.conf:
  file.managed:
    - name: /tmp/brothers.conf
    - source: salt://brothers.jinja
    - template: jinja

brothers.jinja:

{% for key, text in accumulator.items() -%}
    {{ key }}: {{ text|join(', ') }}
{% endfor %}

Steps to Reproduce the behavior

[root@master1 ~]# salt-call --state-output terse state.apply brothers
local:
  Name: uncledonald - Function: file.accumulated - Result: Clean Started: - 12:08:40.767783 Duration: 1.319 ms
  Name: newhart - Function: file.accumulated - Result: Clean Started: - 12:08:40.776429 Duration: 1.363 ms
  Name: /tmp/brothers.conf - Function: file.managed - Result: Changed Started: - 12:08:40.784444 Duration: 29.927 ms

Summary for local
------------
Succeeded: 3 (changed=1)
Failed:    0
------------
Total states run:     3
Total run time:  32.609 ms
[root@master1 ~]# cat /tmp/brothers.conf 
uncledonald: Huey, Dewey, Louie
newhart: Larry, Darryl

Expected behavior

The other brother Darryl should show up in the resulting file:

uncledonald: Huey, Dewey, Louie
newhart: Larry, Darryl, Darryl

Versions Report

Salt Version:
           Salt: 3001.1
 
Dependency Versions:
           cffi: Not Installed
       cherrypy: Not Installed
       dateutil: Not Installed
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
         Jinja2: 2.11.1
        libgit2: Not Installed
       M2Crypto: 0.35.2
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.6.2
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: Not Installed
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 3.6.8 (default, Apr  2 2020, 13:34:55)
   python-gnupg: Not Installed
         PyYAML: 3.13
          PyZMQ: 17.0.0
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.5.3
            ZMQ: 4.1.4
 
System Versions:
           dist: centos 7 Core
         locale: UTF-8
        machine: x86_64
        release: 5.4.0-42-generic
         system: Linux
        version: CentOS Linux 7 Core

Start with the file.accumulated state described in the issue and reproduce the brothers.sls example to confirm that duplicate text is omitted. Trace how the named accumulator handles the text list, then add coverage for repeated values and verify that the generated brothers.conf contains both occurrences of Darryl.