"Attempt to complete a Future more than once" after "Transport is already closed"

[jlokier]

This crash bug happened while running eth1 nimbus without arguments, syncing up to mainnet.
It's rare, it doesn't happen often, but it's a serious logic bug and crashes the program, so worth looking into.

WRN 2021-03-08 15:14:11.720+00:00 Error while handling RLPx message topics="rlpx" tid=625298 file=rlpx.nim:599 peer=Node[104.247.201.82:30303] msg=getBlockHeaders err="Transport is already closed!"
Traceback (most recent call last, using override)
/status/nimbus-eth1/nimbus/nimbus.nim(206) nimbus
/status/nimbus-eth1/nimbus/nimbus.nim(174) process
/status/nimbus-eth1/vendor/nimbus-build-system/vendor/Nim/lib/system/excpt.nim(407) reportUnhandledError
/status/nimbus-eth1/vendor/nimbus-build-system/vendor/Nim/lib/system/excpt.nim(358) reportUnhandledErrorAux
Error: unhandled exception: An attempt was made to complete a Future more than once. Details:
  Future ID: 2050865
  Creation location:
    p2p_protocol_dsl.nim(151)    [unspecified]
  First completion location:
    rlpx.nim(67)    [unspecified]
  Second completion location:
    rlpx.nim(98)    [unspecified]

 [FutureDefect]

[jangko]

when I was implementing EIP2718 and EIP2930 in nim-eth, I also saw some of the test produce this error when running in CI, but forget which one.