Hello and thanks for your response.
By "clobbered", I mean that the original session (and all channels associated with it) stops updating. I also mean that host:port pairs seem to be the internal key for mapping session data structures, as seen below.
As for my configuration, I can prove that it's not germane:
Start emacs with no config.
n.b.: the exact usernames in the following snippets are not important, just that they are different.
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$ emacs -Q
M-x erc RET
6667 RET
testuser1 RET
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So far, so good. We have a session.
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M-x erc RET
6667 RET
testuser2 RET
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You should notice that the 'freenode' buffer has been reused and that the original session cannot be reached. My guess is that this is an original ERC design limitation, but it probably ought to be addressed.