Filter out duplicate values from the variable options dropdown

What does this MR do and why?

This MR adds filtering of duplicate options of the variable options dropdown. In the GlCollapsibleListbox, the value of the listbox item is used as a key, and having duplicated keys causes rendering issues (such as the search not working).

References

• #558525 (closed)

Screenshots or screen recordings

Added in an internal comment

How to set up and validate locally

1. Create a .gitlab-ci.yml file that contains variables with duplicated options.
2. Navigate to Build -> Pipelines -> New and verify the variable options dropdowns work as expected with search functionality working correctly. Verify that the duplicated options are filtered out.

Example of the .gitlab-ci.yml file

variables:
  MODULE_NAME:
    value: ""
    description: "Module name to be built"
    
  N_OPTION:
    value: "option_a"
    description: "Build flavor"
    options:
      - ""
      - "option_a"
      - "option_b"
      - "option_a"  # Duplicate to test deduplication
      
  MODULE_TARGET1:
    value: "-- MUST BE DEFINED --"
    description: "Primary target to build"
    options:
      - "-- MUST BE DEFINED --"
      - "target_1"
      - "target_2"
      - "target_3"
      - "target_4"
      - "target_1"  # Duplicate
      - "target_2"  # Duplicate
      - "target_5"
      - "target_6"
      - "target_5"  # Duplicate
      
  MODULE_TARGET2:
    value: "-- TO BE DEFINED --"
    description: "Secondary target to build"
    options:
      - "-- TO BE DEFINED --"
      - "target_a"
      - "target_b"
      - "target_c"
      - "target_c"  # Duplicate
      - "target_d"
      - "target_b"  # Duplicate
      
stages:
  - build
  
.build_component_script: &build_component_script
  - |
    echo "Building target ${MODULE_TARGET}"
    echo "Module: ${MODULE_NAME}"
    echo "Option: ${N_OPTION}"
    
build1_component:
  tags:
    - test-runner
  stage: build
  rules:
    - if: $MODULE_TARGET1 == "-- MUST BE DEFINED --"
      when: never
    - when: always
  script:
    - export MODULE_TARGET="${MODULE_TARGET1}"
    - *build_component_script
    
build2_component:
  tags:
    - test-runner
  stage: build
  rules:
    - if: $MODULE_TARGET2 == "-- TO BE DEFINED --"
      when: never
    - when: always
  script:
    - export MODULE_TARGET="${MODULE_TARGET2}"
    - *build_component_script

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Related to #558525 (closed)