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This is the code snippet that I am using

$objectManager = MagentoFrameworkAppObjectManager::getInstance();
$quote = $objectManager->get('MagentoCheckoutModelCart');
$_product = $objectManager->get('MagentoCatalogModelProduct')->load(1058);
$params = array (
 'qty' => 1
);
$quote->addProduct($_product, $params);
$quote->save();

$quote = $objectManager->get('MagentoCheckoutModelCart');
$product = null;

$_product = $objectManager->get('MagentoCatalogModelProduct')->load(1059);
$params = array (
 'qty' => 3
);
$quote->addProduct($_product, $params);

$quote->save();

The code seems fine but it updates the quantity of 1st product and results into 4 quantities of first product added to cart.

Please help.

Based on the comments:

You should use

$objectManager->create('MagentoCatalogModelProduct');

instead of

$objectManager->get('MagentoCatalogModelProduct');

This happens because get instantiates a class as a singleton. create always gets you a new instance.

In the code you have, you will get the same instance of the product the second time you try to instantiate it.

Another way, without using Object Manager

$_product = $this->productFactory->create()->load(1059);

Instead of

$_product = $objectManager->get('MagentoCatalogModelProduct')->load(1059);

So, you need to inject MagentoCatalogModelProductFactory in the constructor of class where you are using that snippet