Don't allow picking the same product twice

src/ProductPicker.js

@@ -37,6 +37,10 @@ class ProductPicker extends React.Component {
 
   pickProduct(product) {
     let picks = this.props.picks;
+    if (picks.find((p) => product.code === p.code)) {
+      return;
+    }
+
     picks.push({
       code: product.code,
       name: product.name,