[Bug]: SaltClass duplicate entries in merged list

What happened?

I re-open bug #49175 as it it still available in 3008.2

My setup:

cat classes/test1.yml 
pillars:
  data:
    - data 1 from test1
    - data 2 from test1

cat classes/test2.yml 
pillars:
  data:
    - data 1 from test2
    - data 2 from test2

cat nodes/minion.yml
environment: base

classes:
  - test1
  - test2

Checking minion's pillars:

salt 'minion' pillar.items unmask=true
minion:
    ----------
    __saltclass__:
        ----------
        classes:
            - test1
            - test2
        environment:
            base
        nodename:
            minion
        states:
    data:
        - data 1 from test1
        - data 2 from test1
        - data 1 from test2
        - data 2 from test2
        - data 1 from test2
        - data 2 from test2

Pillars from the second class are still doubled.

Type of salt install

Official deb

Major version

3008.x

What supported OS are you seeing the problem on? Can select multiple. (If bug appears on an unsupported OS, please open a GitHub Discussion instead)

debian-13

salt --versions-report output

Salt Version:
          Salt: 3008.2
 
Python Version:
        Python: 3.14.6 (main, Jun 11 2026, 02:19:05) [GCC 11.2.0]
 
Dependency Versions:
          cffi: 2.0.0
      cherrypy: 18.10.0
  cryptography: 48.0.0
      dateutil: 2.9.0.post0
     docker-py: Not Installed
         gitdb: 4.0.12
     gitpython: 3.1.50
        Jinja2: 3.1.6
       libgit2: Not Installed
  looseversion: 1.3.0
      M2Crypto: Not Installed
          Mako: Not Installed
       msgpack: 1.1.2
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     packaging: 24.0
     pycparser: 3.00
      pycrypto: Not Installed
  pycryptodome: 3.23.0
        pygit2: Not Installed
  python-gnupg: 0.5.6
        PyYAML: 6.0.3
         PyZMQ: 27.1.0
        relenv: 0.22.14
         smmap: 5.0.2
       timelib: 0.3.0
       Tornado: 6.5.7
           ZMQ: 4.3.5
 
Salt Package Information:
  Package Type: onedir
 
System Versions:
          dist: debian 13.6 trixie
        locale: utf-8
       machine: x86_64
       release: 6.12.101+deb13-amd64
        system: Linux
       version: Debian GNU/Linux 13.6 trixie

Reproduce the issue with classes/test1.yml, classes/test2.yml, and nodes/minion.yml, then run salt 'minion' pillar.items unmask=true on Salt 3008.2. Trace how the two classes are merged and add a focused regression test for the duplicated second-class entries. Done means each listed pillar value appears once in the merged result.