Description of problem:

We sometimes observe that an instance has duplicated attachments for the same volume,
while the volume has also duplicated attachments for the instance.

This causes wrong handling of block devices in nova, and causes issue in the instance,
or the failure when rebooting the instance caused by duplicated lock about the same volume

How reproducible:
It doesn't always happen, but sometime does.
We observe that it tends to happen when ocp on top of osp continuously reties to attach a volume to an instance

Steps to Reproduce:
1. Create volume A
2. Create instance B
3. Attach a volume A to instance B concurrently

Actual results:
The instance B sometimes gets duplicated attachments for the volume A


Expected results:
The instance B never gets duplicated attachments for the volume A


Additional info: