Description of problem:

Migration of a cinder volume (accidently to the same share) will delete the volume

Where are you experiencing the behavior?  What environment?

while testing the migration of a cinder volume from one NFS Share to another, we encountered the problem, that after submitting the same
"openstack volume migrate" twice, or migrate a volme to the same share the specified volume is gone (deleted?).

When does the behavior occur? Frequently?  Repeatedly?   At certain times?

For example:
We create a new cinder volume:
(overcloud) root@d100siulxosds31 ~ # openstack volume create --size 1 --type 'cinder-BEb' nfsvol1
+---------------------+--------------------------------------+
| Field               | Value                                |
+---------------------+--------------------------------------+
| attachments         | []                                   |
| availability_zone   | nova                                 |
| bootable            | false                                |
| consistencygroup_id | None                                 |
| created_at          | 2019-12-16T13:19:26.000000           |
| description         | None                                 |
| encrypted           | False                                |
| id                  | c3706070-0918-4493-9d1f-4ff7ebdf6ae4 |
| migration_status    | None                                 |
| multiattach         | False                                |
| name                | nfsvol1                              |
| properties          |                                      |
| replication_status  | None                                 |
| size                | 1                                    |
| snapshot_id         | None                                 |
| source_volid        | None                                 |
| status              | creating                             |
| type                | cinder-BEb                           |
| updated_at          | None                                 |
| user_id             | 9cee1efa4e6547069645109b1c3aa7cd     |
+---------------------+--------------------------------------+
(overcloud) root@d100siulxosds31 ~ #

We see the new volume inside the specified cinder share:
()[root@d100siulxoscs31 4f07fd98d192255657be0913c997506a]# ls -al
total 12
drwxr-xr-x. 3 cinder kolla       4096 Dec 16 14:19 .
drwxr-sr-x. 4 root   kolla       4096 Dec 16 14:09 ..
drwxr-xr-x. 2 cinder kolla       4096 Dec 16 14:16 new_filer
-rw-rw-rw-. 1 root   root  1073741824 Dec 16 14:19 volume-c3706070-0918-4493-9d1f-4ff7ebdf6ae4
()[root@d100siulxoscs31 4f07fd98d192255657be0913c997506a]#

And we see the new volume in the output of the openstack command:
(overcloud) root@d100siulxosds31 ~ # openstack volume show nfsvol1
+--------------------------------+--------------------------------------+
| Field                          | Value                                |
+--------------------------------+--------------------------------------+
| attachments                    | []                                   |
| availability_zone              | nova                                 |
| bootable                       | false                                |
| consistencygroup_id            | None                                 |
| created_at                     | 2019-12-16T13:19:26.000000           |
| description                    | None                                 |
| encrypted                      | False                                |
| id                             | c3706070-0918-4493-9d1f-4ff7ebdf6ae4 |
| migration_status               | None                                 |
| multiattach                    | False                                |
| name                           | nfsvol1                              |
| os-vol-host-attr:host          | hostgroup@cinder-BEb#NFS_BEb         |
| os-vol-mig-status-attr:migstat | None                                 |
| os-vol-mig-status-attr:name_id | None                                 |
| os-vol-tenant-attr:tenant_id   | 2bfdd67ba38248c7bf89e2ed6683138b     |
| properties                     |                                      |
| replication_status             | None                                 |
| size                           | 1                                    |
| snapshot_id                    | None                                 |
| source_volid                   | None                                 |
| status                         | available                            |
| type                           | cinder-BEb                           |
| updated_at                     | 2019-12-16T13:19:33.000000           |
| user_id                        | 9cee1efa4e6547069645109b1c3aa7cd     |
+--------------------------------+--------------------------------------+
(overcloud) root@d100siulxosds31 ~ #

We migrate the volume, accidently to the same share, or we execute the same migration command to another share twice:
(overcloud) root@d100siulxosds31 ~ # openstack volume migrate --host 'hostgroup@cinder-BEb' nfsvol1

For a shot period of time we see two volumes:
()[root@d100siulxoscs31 4f07fd98d192255657be0913c997506a]# ls -al
total 12
drwxr-xr-x. 3 cinder kolla       4096 Dec 16 14:21 .
drwxr-sr-x. 4 root   kolla       4096 Dec 16 14:09 ..
drwxr-xr-x. 2 cinder kolla       4096 Dec 16 14:16 new_filer
-rw-rw-rw-. 1 root   root  1073741824 Dec 16 14:21 volume-1b632ca4-7848-4b57-be06-53f47506b9f4
-rw-rw-rw-. 1 root   root  1073741824 Dec 16 14:19 volume-c3706070-0918-4493-9d1f-4ff7ebdf6ae4
()[root@d100siulxoscs31 4f07fd98d192255657be0913c997506a]#

And then they disappear both:
()[root@d100siulxoscs31 4f07fd98d192255657be0913c997506a]# ls -al
total 12
drwxr-xr-x. 3 cinder kolla 4096 Dec 16 14:22 .
drwxr-sr-x. 4 root   kolla 4096 Dec 16 14:09 ..
drwxr-xr-x. 2 cinder kolla 4096 Dec 16 14:16 new_filer
()[root@d100siulxoscs31 4f07fd98d192255657be0913c997506a]#

Openstack still sees the volume, which ist now gone:
(overcloud) root@d100siulxosds31 ~ # openstack volume show nfsvol1
+--------------------------------+--------------------------------------+
| Field                          | Value                                |
+--------------------------------+--------------------------------------+
| attachments                    | []                                   |
| availability_zone              | nova                                 |
| bootable                       | false                                |
| consistencygroup_id            | None                                 |
| created_at                     | 2019-12-16T13:19:26.000000           |
| description                    | None                                 |
| encrypted                      | False                                |
| id                             | c3706070-0918-4493-9d1f-4ff7ebdf6ae4 |
| migration_status               | success                              |
| multiattach                    | False                                |
| name                           | nfsvol1                              |
| os-vol-host-attr:host          | hostgroup@cinder-BEb#NFS_BEb         |
| os-vol-mig-status-attr:migstat | success                              |
| os-vol-mig-status-attr:name_id | None                                 |
| os-vol-tenant-attr:tenant_id   | 2bfdd67ba38248c7bf89e2ed6683138b     |
| properties                     |                                      |
| replication_status             | None                                 |
| size                           | 1                                    |
| snapshot_id                    | None                                 |
| source_volid                   | None                                 |
| status                         | available                            |
| type                           | cinder-BEb                           |
| updated_at                     | 2019-12-16T13:22:09.000000           |
| user_id                        | 9cee1efa4e6547069645109b1c3aa7cd     |
+--------------------------------+--------------------------------------+
(overcloud) root@d100siulxosds31 ~ #

Version-Release number of selected component (if applicable):
OSP13

How reproducible:
always

Steps to Reproduce:
1. create a cinder volume on a nfs share 
2. migrate the volume to the same share
3.

Actual results:
Volume deleted

Expected results:
for example forbid to migrate to the same share

Additional info: