Canceled invoice can be canceled again

[redelschaap]

A canceled invoice can be canceled again. I think \Magento\Sales\Model\Order\Invoice::cancel should check whether the invoice can be canceled before executing all cancel operations.

Preconditions

1. Tested on Magento 2.1.5

Steps to reproduce

1. Load a canceled invoice programmatically
2. Call $invoice->cancel()

Expected result

1. Throw an exception saying the invoice has already been canceled.

Actual result

1. The invoice gets canceled twice, so order amounts are also being adjusted twice.