I would like to know how can I avoid having the user re-pay for an in-app subscription he already purchased.
Example: I purchase an App which offers in-app purchase on google play store for 5$ and now I shift from using an Android phone to an iPhone.
I came across this post which says it's not possible as they are two different competitors and would have to re-purchase the same app on the iOS once again, but is there a way to avoid it so that the end-user does not repay the sum of 5$ again?
I was thinking if it would be a good idea to have the receipt stored on a server and then when the user enters his email just do a lookup if there's a receipt associated with that email along with some validation logic.
In theory, I could avoid the user having to pay the 5$ with this approach but at the same time, I am not sure if doing this could get my app rejected or If this could lead to some different issues OR this is not doable at all and the user will have to end up paying 5$ anyways if he shifts from android to ios and vice versa.
The approach you suggested is correct. You would store the receipt on your server, along with the user identifier. Your server is responsible for refreshing the receipt with Apple/Google to keep the status up-to-date. You can then have an endpoint to your backend to check if a user is subscribed - regardless of the platform.
This is well within the App Store Guidelines, and is how large subscription businesses like Netflix and Spotify operate.