Magento 2: Prevent add to cart or qty increase if a product already exists in cart

You can set it by Magento 2 configuration :

Store -> Configuration -> Catalog -> Inventory -> Product Stock Options -> Maximum Qty Allowed in Shopping Cart

Step 1 -
Create events.xml :

<event name="controller_action_predispatch_checkout_cart_add">
  <observer name="vendor_model_cart_add_before" instance="VendorModuleObserverCartAddBefore" />
</event>

Step 2 -
Create CartAddBefore.php :

<?php
namespace VendorModuleObserver;

use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkAppRequestDataPersistorInterface;
use MagentoFrameworkAppObjectManager;

class CartAddBefore implements ObserverInterface {

    protected $_cart;
    protected $_messageManager;
    protected $_request;


    public function __construct(
        MagentoCheckoutModelCart $cart,
        MagentoFrameworkMessageManagerInterface $messageManager,
        MagentoFrameworkAppRequestInterface $request,
    ) {
        $this->_cart = $cart;
        $this->_messageManager = $messageManager;
        $this->_request = $request;        
    }

    public function execute(MagentoFrameworkEventObserver $observer) {

        $postValues = $this->_request->getPostValue();
        $cartItemsCount = $this->_cart->getQuote()->getItemsCount();

        if($cartItemsCount > 1)
        {
            ...... your logic .......
            $this->_messageManager->addError(__('Sorry, Only 1 product is allowed'));
        }
    }
}

You can do that with default Magento setup.

In Magento Admin, go to

For all Products

Stores > Configuration > Catalog > Inventory > Product Stock Options.

For individual Product

Catalog > Click on Product from Grid > Advanced Inventory

Set Maximum Qty Allowed in Shopping Cart to 1. This will restrict allowing more than 1 item in the shopping cart.

Hope it was helpful. Thanks.