Although a simple solution the recently introduced approach to just not listen to incoming connetions if there's already one has the downside that one doesn't know if a connection fails because the server is down or becuase there's a connection.
Therefore now connections are always accepted but after sending a message to the user and waiting a few seconds they are closed.
The appstate is used used in a somewhat unusual way here: The one and only "active" connection has an appstate of 0. For all other connections the appstate is initially set to 1. Then it is increased on every uIP poll until it reaches 10, which triggers the connection close. This somewhat hacky approach allows to keep track of the rejected sessions without any additional state variables and/or timers.
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