Welcome!

compact version tested on Odoo 19.0:

MAILING_LIST_ID = 1  # malinglist ID
for p in records.filtered('email'):
    if env['mailing.contact'].search([('email', '=', p.email), ('list_ids', 'in', MAILING_LIST_ID)], limit=1):
        continue
    env['mailing.contact'].create({
        'name': p.name,
        'email': p.email,
        'country_id': p.country_id.id or False,
        'list_ids': [(4, MAILING_LIST_ID)]
    })

• activate the developer mode and go to settings->technical-> server actions
• create the following action and contextual action (using button) to make it available under actions from the contact app (use list view to select several/all contacts):

Code:

# Define the target mailing list, as an action should be created per mailing list
TARGET_LIST = 'xyz'
target_list = env['mailing.list'].search([('name', '=', TARGET_LIST)], limit=1)
mailing_contacts = env['mailing.contact'].search([('list_ids.name', '=', TARGET_LIST)])
for rec in records:
    # Ensure email exists
    if rec.email:
        # Check if email already exists in the target mailing list
        if not any(contact.email == rec.email for contact in mailing_contacts):
            # Create new contact if not duplicate
            new_contact = env['mailing.contact'].create({
                'name': rec.name,
                'email': rec.email,
                'title_id': rec.title.id if rec.title else False,
                'company_name': rec.parent_name if rec.parent_name else False,
                'country_id': rec.country_id.id if rec.country_id else False,
                'tag_ids': [(6, 0, rec.category_id.ids)] if rec.category_id else False,
            })
            # Subscribe to the target mailing list (if the list exists)
            if target_list:
                new_contact.write({'list_ids': [(4, target_list.id)]})
action = {'type': 'ir.actions.act_window_close'}