// original source http://stackoverflow.com/a/35072168

function itl_is_product_the_same_cat( $valid, $product_id, $quantity ) {
    global $woocommerce;

    foreach ( $woocommerce->cart->get_cart() as $cart_item_key => $values ) {
        $_product 	  = $values['data'];
        $terms 		  = get_the_terms( $_product->id, 'product_cat' );
        $target_terms = get_the_terms( $product_id, 'product_cat' ); // get the current items

        foreach ( $terms as $term ) {
            $cat_ids[] = $term->term_id;  // get all the item categories in the cart
        }
        foreach ($target_terms as $term) {
            $target_cat_ids[] = $term->term_id; // get all the categories of the product
        }
    }

	if ( ! empty ( WC()->cart->get_cart() ) && $valid ) {
		$same_cat = array_intersect( $cat_ids, $target_cat_ids ); // check if they have the same category

		if ( count( $same_cat ) == 1 ) {
			wc_add_notice( 'You can only have 1 item per category in your cart', 'error' );
			return false;
		} else {
			return $valid;
		}
	} else {
		return $valid;
	}
}
add_filter( 'woocommerce_add_to_cart_validation', 'itl_is_product_the_same_cat', 10, 3 );