Unable to accept a shared mailbox folder when already one has been accepted

When accepting a shared mailbox folder from an user and the user already accepted a shared mailbox folder from another user, the following error will be displayed in a new window:
You have already installed this shared folder.

The new shared folder cannot be accepted.

STEPS TO REPRODUCE (Enough detail to create a test case):

1. create a PO with 3 users: USER1, USER2 and USER3
2. login as USER1 and share the mailbox (folder) with USER3. Logout
3. login as USER2 and share the mailbox (folder) with USER3. Logout
4. login as USER3.
5. open the first "Shared Folder Notification" and accept.
6. open the second "Shared Folder Notification" and the following error will be displayed: You have already installed this shared folder.