I am using Amazon in-app Purchase API. I have this item that is Entitlement type, but the user has to buy it more than once and it cannot be Consumable type because it has to be available on all the devices. How can I change the message "You already own the item..."?
The "You already own the item..." message is expected when you try to purchase an entitlement which you already own. It is by design and you cannot change it.
According to Amazon IAP API:
Consumables - content that does not require an entitlement or access rights to use, can be purchased multiple times by a customer, is available only on the device it is purchased from, and does not require any type of entitlement check to use
Entitlements - content that requires an entitlement or access rights to use, can be purchased only once by a customer, and is available on all compatible devices registered to the customer's account
So neither consumable nor entitlement matches your use case directly.
If your item is used like a power-up, then you should make it a consumable, and implement your own mechanism to sync the available item count across devices. If you are developing a game, you could use Amazon Whispersync for Games API to sync those data.
If your item is used to unlock certain features for a limited time, then you should make it a subscription.
Subscriptions - content that requires an entitlement or access rights to use, is bound by a period of time, that auto-renews, and is available on all eligible devices registered to the customer's Amazon account