For a system currently being constructed, the user will need to log in to multiple systems with the same password. One system (A) is always leading, and the user can only change the password on system A. The new hash is then copied to system B and on. Each system B only has a subset of the credentials of system A.
For technical (deployment and connectivity) reasons we cannot use a single sign-on technology like OAuth to transfer trust. The systems B are rarely connected, so while we can push information from A to B, we cannot allow B to rely on a connection to A for its user validation. Of course we correctly salt and hash the passwords.
Now I wonder: is it bad practice for these systems, all of them under our control, to share the same password hash for a user?