Specify that product display names must be unique within an app (#303)

## Motivation / Description
* Specify that product display names must be unique within an app

https://linear.app/revenuecat/issue/DENG-496/product-identifier-already-exists

## Changes introduced
## Jira ticket (if any)
## Additional comments

docs_source/🚀 Getting Started/entitlements.md

@@ -83,6 +83,10 @@ To make it easier to identify your products in RevenueCat, you can optionally se
 3. Entering your desired display name
 4. Clicking **Save**
 
+> 📘 
+> 
+> Product display names must be unique within an app. It's a good practice to include the product duration in the display name to avoid name overlaps, for example: 'My Pro Subscription Monthly', 'My Pro Subscription Yearly' etc".
+
 > 🚧 
 > 
 > Product display names are currently supported in Charts, and will be supported throughout the Dashboard in the near future.