It may say that, but it won't actually charge you - many of us have been moaning about how this is presented for years.
With the caveat that you are doing the right thing. A few customers have bought the game from the store then at a later date downloaded the free one and tried to unlock via the iap. Those ARE different products and you WILL be charged again, there's nothing we can do to help that. But if you are definitely rebuying the same identical thing from the same app, you will NOT be charged again.
Hope that helps and thanks for coming back.
Regards, Paul Johnson.