_isActivePlayer is an internal function so it can only be called by this contract or contracts that inherit it, and it only checks whether msg.sender is in the players array. But because this function is internal and not called by any other functions in the contract, the only address that could be msg.sender is this contract, and this contract isn't gonna be a player in the lottery.
This function serves no purpose in its current state. No active players can actually call it. If the purpose is for someone to be able to confirm they are in the current lottery, it should be an external function.
Manual review
Make it an external function. It does have a purpose in allowing people to confirm they have been entered in the lottery.
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