Are these links in the same course or different courses?
If they are in different courses, they will be in different contexts and by design we need to create a different user. Here's why: if you have one course where a user is a teacher and another where the same user is a student, then what role will their account have? They are in a different context so must be treated differently.
I created an issue on tracker for you: https://tracker.moodle.org/browse/MDL-55905 But please modify it with more details if you can. I don't have a full grasp on the problem you are experiencing yet.
This might be because blackboard and moodle give a different context for different instances or it might be a problem in the provider code, but we can work that out while working on https://tracker.moodle.org/browse/MDL-55905