Reporter didn't provide a version. Using the latest from master, I was not able to reproduce this issue (see below). Since the reporter mentioned that Cinder does not know about this false attachment, but Nova does, I would bet something is being set on the Nova side.

ubuntu@mount-issue:~/devstack$ nova list
c+--------------------------------------+---------+--------+------------+-------------+------------------+
| ID                                   | Name    | Status | Task State | Power State | Networks         |
+--------------------------------------+---------+--------+------------+-------------+------------------+
| 57304c45-101d-4ce0-8f4b-6b7ad853d135 | server1 | ACTIVE | -          | Running     | private=10.0.0.2 |
| 1cb270bd-2131-42fa-9f99-ed95cd077cde | server2 | ACTIVE | -          | Running     | private=10.0.0.3 |
+--------------------------------------+---------+--------+------------+-------------+------------------+
iubuntu@mount-issue:~/devstack$ cinder list
+--------------------------------------+-----------+------+------+-------------+----------+-------------+
|                  ID                  |   Status  | Name | Size | Volume Type | Bootable | Attached to |
+--------------------------------------+-----------+------+------+-------------+----------+-------------+
| 2a57f161-0828-4b68-8f93-cd4493ff725b | available | None |  1   | lvmdriver-1 |  false   |             |
+--------------------------------------+-----------+------+------+-------------+----------+-------------+
ubuntu@mount-issue:~/devstack$ nova volume-attach server1 2a57f161-0828-4b68-8f93-cd4493ff725b
+----------+--------------------------------------+
| Property | Value                                |
+----------+--------------------------------------+
| device   | /dev/vdb                             |
| id       | 2a57f161-0828-4b68-8f93-cd4493ff725b |
| serverId | 57304c45-101d-4ce0-8f4b-6b7ad853d135 |
| volumeId | 2a57f161-0828-4b68-8f93-cd4493ff725b |
+----------+--------------------------------------+
ubuntu@mount-issue:~/devstack$ nova volume-attach server2 2a57f161-0828-4b68-8f93-cd4493ff725b
ERROR (BadRequest): Invalid volume: volume '2a57f161-0828-4b68-8f93-cd4493ff725b' status must be 'available'. Currently in 'in-use' (HTTP 400) (Request-ID: req-bfa40f00-56f9-4535-a4d1-23a8cb69b908)
ubuntu@mount-issue:~/devstack$ nova volume-attach server2 2a57f161-0828-4b68-8f93-cd4493ff725b
ERROR (BadRequest): Invalid volume: volume '2a57f161-0828-4b68-8f93-cd4493ff725b' status must be 'available'. Currently in 'in-use' (HTTP 400) (Request-ID: req-83736950-ff20-4d93-b512-ab7ffc25c7bc)
ubuntu@mount-issue:~/devstack$ cinder list
+--------------------------------------+--------+------+------+-------------+----------+--------------------------------------+
|                  ID                  | Status | Name | Size | Volume Type | Bootable |             Attached to              |
+--------------------------------------+--------+------+------+-------------+----------+--------------------------------------+
| 2a57f161-0828-4b68-8f93-cd4493ff725b | in-use | None |  1   | lvmdriver-1 |  false   | 57304c45-101d-4ce0-8f4b-6b7ad853d135 |
+--------------------------------------+--------+------+------+-------------+----------+--------------------------------------+
ubuntu@mount-issue:~/devstack$ http http://localhost:8774/v2/b9903b32e6e94a5ab2ee40e217be0fab/servers/1cb270bd-2131-42fa-9f99-ed95cd077cde/os-volume_attachments X-Auth-Token:'e08bc40fd5f148cf817f99c1133da8aa'
HTTP/1.1 200 OK
Content-Length: 25
Content-Type: application/json
Date: Fri, 02 Jan 2015 18:37:27 GMT
X-Compute-Request-Id: req-5d213be2-60af-450a-b521-8f979d5a80e6

{
    "volumeAttachments": []
}