You're right! That is a super cool idea. It would let you spot a user re-using passwords and warn them not to. However, there might be some complexities. This

You're right! That is a super cool idea. It would let you spot a user re-using passwords and warn them not to. However, there might be some complexities. This is basically poor-password-handling as a well-intentioned service. It means anyone breaching any of the services the hub knows about would let you see a great many username/password pairings being checked. It would take make passwords as safe as the weakest link across all the services being checked. The odds of N services all being absolutely safe all at once aren't as good as those of a single service. This is why people shouldn't re-use passwords, right? I can see your heart is in the right place. You're looking to protect users from themselves! That's a wonderful thing, and I'm so glad that there are people like you who care. But the above outcome might not be maximally desirable, you know?