Synapse only maintains a threepid's relationship with zero or one identity servers, but I think it is (theoretically) possible for it one threepid to be associated with many identity servers

[lampholder]

@bob:homeserver.computer could:

• fire up a Riot with the IS alpha.im
• publish the threepid bob@gmail.com to alpha.im
• fire up another Riot with the IS beta.im
• publish bob@gmail.com to beta.im

Then I expect we'd lose the assocation with alpha.im, meaning the HS could no longer tidy up the threepid binding on account close.