Using a JS Set and Array.flat() to get only the unique values from many arrays.

mergeUniqueFlatten.js

const clients = [
    {
      name: 'Client A',
      services: ['Web development', 'SEO', 'Web design', 'Full service agency'],
    },
    {
      name: 'Client B',
      services: [
        'Analytics', 'Email marketing', 'Marketing', 'SEO', 'Web design', 'Web development', 'Integration solutions', 'Full service agency','Advertising',
      ],
    },
  ];
const uniqueServices = [
  ...new Set(clients.map(({services}) => services).flat()),
];