A single google account can have both monthly and yearly subscriptions

If you create two Pocket Casts accounts on a single device, it is possible to separately upgrade both of those accounts to Plus on that device using the same google account as long as you make one a monthly subscription and one an annual subscription.

I think we probably need to tweak our subscription setup in the Google Play Store to make it so that these two subscriptions are mutually exclusive. I can't think of any reason we would want to allow a single account to have both a monthly and an annual subscription.

Or, if we do want to allow a single account to have multiple subscriptions, then we need to find a way to allow that when the subscription type is the same. Currently, a single account cannot have two monthly or two yearly subscriptions. Instead, Google will reject the purchase because you already have a subscription and the Pocket Casts server will move the subscription from the Pocket Casts account that originally subscribed to the new Pocket Casts account that is trying to subscribe.

### Test Steps
#### Different subscriptions
1. Pay for a Plus subscription on an Android phone through Google Play.
2. Sign out of the Pocket Casts account that was just upgraded
3. Create a new account
4. Go through the payment flow to buy a second subscription of **_a different_** type (if the first subscription was monthly, make this one yearly).
5. Google adds a second subscription to the active Google account.

#### Same subscription
1. Pay for a Plus subscription on an Android phone through Google Play.
2. Sign out of the Pocket Casts account that was just upgraded
3. Create a new account
4. Go through the payment flow to buy a second subscription of **_the same_** type (if the first subscription was monthly, make this one monthly as well).
5. There is an error message saying that this Google account is already subscribed. The second Pocket Casts account will now have a Plus subscription but the first Pocket Casts account will no longer have a Plus subscription.

Start with the issue's Google Play reproduction steps and inspect the Android subscription setup and purchase-handling entry points. Determine whether subscriptions of different types must be mutually exclusive or whether same-type subscriptions should be supported; done means the selected policy is enforced without moving Plus access between Pocket Casts accounts.

Written by the indexing model from the issue text.