After reinstal you will get new RegId and prev not valid anymore. So, even if you send push to both RegIds, only last will received it.
You can implement logic for accounts in application.
For example, when user login in application you send his GoogleId + RegId. After reinstall of application and relogin you just update RegId on server. So, you can have only one RegId for every user.
There is problem: only one device will receive push msg (if you login in 2 devices with same account). So, you can send to server GoogleId + RegId + DeviceId after start application.
