Merge two diffferent Drupal 7 vocabularies into a single Drupal 8/9 vocabulary and dedupe terms

dme_d7_taxonomy_term_tags.yml

id: dme_d7_taxonomy_term_tags
migration_group: dmemigration
label: 'Taxonomy terms (Tags & Blog tags)'

source:
  plugin: d7_taxonomy_term
  # We're merging the D7 blog_tags, and tags taxonomies into one.
  bundle:
    - tags
    - blog_tags

process:
  tid:
    -
      # First check to see if there's already a term with the
      # same name. If so, use it's TID. Effectively deduping
      # terms. For duplicate terms this will create two records
      # in the map table both of which point to the same destid1
      # so using migration_lookup will work regardless of which
      # of the two terms the migrated content used.
      plugin: entity_lookup
      source: name
      value_key: name
      bundle_key: vid
      bundle: tags
      entity_type: taxonomy_term
      ignore_case: true
    -
      # If not, use next available TID.
      plugin: default_value
      default_value: null
  vid:
    -
      # Force all terms to use same vocab, regardless of
      # source vocab.
      plugin: default_value
      default_value: 'tags'
  name:
    -
      plugin: get
      source: name
  description/value:
    -
      plugin: get
      source: description
  description/format:
    -
      plugin: get
      source: format
  parent:
    -
      plugin: default_value
      default_value: 0
  changed:
    -
      plugin: get
      source: timestamp
  langcode:
    -
      plugin: get
      source: language

destination:
  plugin: 'entity:taxonomy_term'
  default_bundle: tags

migration_dependencies:
  required: []
  optional: []

snippet.yml

# Snippet from blog post migration showing how to move data from D7
# blog_tags field to new D8 tags field using a migration_lookup.
process:
  taxonomy_tags:
    -
      plugin: sub_process
      source: taxonomy_blog_tags
      process:
        target_id:
          -
            plugin: migration_lookup
            migration: dme_d7_taxonomy_term_tags
            source: tid