Overwrite the way the friendly_id gem generates unique slugs.

user.rb

  friendly_id :slug_candidates, use: :slugged, slug_column: :namespace

  def slug_candidates
    [
      [:first_name, :last_name]
    ]
  end

  def normalize_friendly_id(value)
    super.gsub(/\-/, '')
  end

  def resolve_friendly_id_conflict(candidates)
    [candidates.first, User.where(namespace: candidates.first).count].compact.join(friendly_id_config.sequence_separator)
  end