I'm using AngarTheme but it's not a theme problem but a code problem ... in practice the additions in bold I made to the product-add-to-cart.tpl file remove the word "Not available" when a user has the latest product in cart but unfortunately adds the double word "Available Available" when a product is available:
{$in_cart = 0}
{foreach from=$cart['products'] item='cart_product'}
{if $cart_product['id_product'] == $product.id}
{$in_cart = 1}
{/if}
{/foreach}
{block name='product_availability'}
<span id="product-availability">
{if $product.show_availability && $product.availability_message}
{if $product.availability == 'available'}
<i class="material-icons rtl-no-flip product-available"></i>
{$product.availability_message}
{elseif $product.availability == 'last_remaining_items'}
<i class="material-icons product-last-items"></i>
{$product.availability_message}
{else}
{if !$in_cart}
<i class="material-icons product-unavailable"></i>
{/if}
{/if}
{if !$in_cart}
{$product.availability_message}
{/if}
{/if}
</span>
{/block}
Best regards,
Fabrizio.