They are easily detectable if the developers have dedicated a part of their software on defending the user from such attacks. Bitcoin Core has worked on it, for example. The user cannot be expected to use all sort of coins, from different devices, for the sake of confirming they aren't under sybil attack. I haven't found anything substantial in their client's repository.

I still don't get it, though. What does it matter? Alice and Bob are two separate Tor identities.