code: function overwriteExistingPurchases( existingPurchases, newPurchases ) {
  	let purchases = newPurchases;
  
  	existingPurchases.forEach( purchase => {
  		if ( ! find( purchases, { ID: purchase.ID } ) ) {
  			purchases = purchases.concat( purchase );
  		}
  	} );
  
  	return purchases;
  }
docs: Overwrites the purchases in the store with the purchases from the new purchases array
  that share the same `id` value.
  
  @param {array} existingPurchases - an array of purchases in the store
  @param {array} newPurchases - an array of purchases fetched from the API
  @return {array} An array of purchases