Some people seem to view CIDR as an opportunity to subdivide their traditional 'Class B' networks. I would classify most of these announcements as 'mistakes' because they usually also announce the supernet, and have the same path.
I could see the intentional announcement of both if you've got diverse -- but ultimately equal -- paths into your B. So site X might advertise B.0/17 to normally receive the low half of the address range, site Y might advertise B.128/17 to normally receive the high half, and both sites would advertise B/16 so that if one site went down in that scenario, the other's B/16 advertisement would take over for the failed site's /17. In a normal case, though, you'd see the /16 and /17 with the same path. Does this pattern seem to apply to any of the announcements you see? dp