Note to for self verification steps see:
https://bugzilla.redhat.com/show_bug.cgi?id=1705563#c0

Verified on:
openstack-cinder-14.0.4-0.20200107100455.a59c01e.el8ost.noarch

First test, try to attach same volume more than once to same instance

First volume gets attached:
(overcloud) [stack@undercloud-0 ~]$ cinder list
+--------------------------------------+--------+------+------+-------------+----------+--------------------------------------+
| ID                                   | Status | Name | Size | Volume Type | Bootable | Attached to                          |
+--------------------------------------+--------+------+------+-------------+----------+--------------------------------------+
| 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 | in-use | -    | 1    | tripleo     | false    | 908ae406-b466-469c-8586-83ecf81101c0 |
+--------------------------------------+--------+------+------+-------------+----------+--------------------------------------+


(overcloud) [stack@undercloud-0 ~]$ for i in {1..10}; do openstack server add volume inst1 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 & done
[1] 361372
[2] 361373
[3] 361374
[4] 361375
[5] 361376
[6] 361377
[7] 361378
[8] 361379
[9] 361380
[10] 361381
(overcloud) [stack@undercloud-0 ~]$ Invalid volume: volume 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 already attached (HTTP 400) (Request-ID: req-4ccf7db2-cbb5-4b8b-ab65-ca01c2635db5)
Invalid volume: volume 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 already attached (HTTP 400) (Request-ID: req-e26e211b-5b8b-4403-94a4-8c7d44535a24)
Invalid volume: volume 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 already attached (HTTP 400) (Request-ID: req-e6ae511b-3fe8-4ead-9584-673c70914a71)
Invalid volume: volume 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 already attached (HTTP 400) (Request-ID: req-08583805-baf1-47a6-bb6b-b581872ef902)
Invalid volume: volume 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 already attached (HTTP 400) (Request-ID: req-c35b9962-6a99-4a8f-ae2b-4709ba891415)
Invalid volume: volume 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 already attached (HTTP 400) (Request-ID: req-d9f7f571-fb86-4e8e-9d7b-7c224a013bda)
Invalid volume: volume 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 already attached (HTTP 400) (Request-ID: req-d33f69e1-af9d-41ee-92e1-27b1df8b58b1)
Invalid volume: volume 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 already attached (HTTP 400) (Request-ID: req-d7508f62-9731-49f5-9c98-744f9ae947f7)
Invalid volume: volume 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 already attached (HTTP 400) (Request-ID: req-3695bcc8-34f3-4417-b1a1-68dc2b81e612)
Invalid volume: volume 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 already attached (HTTP 400) (Request-ID: req-8e482044-d596-4a92-ac92-0f1fe4c14087)


Can't reattach an already attached volume - great. 
Now lets clone the original customer script that hit this. 


Customer's script adapted to my own env:

(overcloud) [stack@undercloud-0 ~]$ cat test.sh 
#!/usr/bin/bash
source /home/stack/overcloudrc
NOVA_URL=http://172.17.1.34:8774/v2.1
INSTANCE_ID=908ae406-b466-469c-8586-83ecf81101c0
VOLUME_ID=88820157-a5f2-44ac-acb2-dde0d8ee4fc1
function call {
        curl -s -H "X-Auth-Token: $OS_TOKEN" -H "X-Subject-Token: $OS_TOKEN" "$@" | jq .
}
echo "Using instance_id: $INSTANCE_ID"
echo "Using volume_id: $VOLUME_ID"
echo
echo "Attachments before test:"
call $NOVA_URL/servers/$INSTANCE_ID/os-volume_attachments
echo
echo "Attempting 10 concurrent attachments..."
for i in {1..10}
do
  call -H 'Content-Type: application/json' $NOVA_URL/servers/$INSTANCE_ID/os-volume_attachments -d "{\"volumeAttachment\": {\"volumeId\": \"$VOLUME_ID\"}}" > /dev/null &
done
sleep 15
echo
echo "Attachements after test:"
call $NOVA_URL/servers/$INSTANCE_ID/os-volume_attachments



(overcloud) [stack@undercloud-0 ~]$ nova volume-attach 908ae406-b466-469c-8586-83ecf81101c0 88820157-a5f2-44ac-acb2-dde0d8ee4fc1
+----------+--------------------------------------+
| Property | Value                                |
+----------+--------------------------------------+
| device   | /dev/vdb                             |
| id       | 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 |
| serverId | 908ae406-b466-469c-8586-83ecf81101c0 |
| tag      | -                                    |
| volumeId | 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 |
+----------+--------------------------------------+
(overcloud) [stack@undercloud-0 ~]$ ./test.sh 
Using instance_id: 908ae406-b466-469c-8586-83ecf81101c0
Using volume_id: 88820157-a5f2-44ac-acb2-dde0d8ee4fc1

Attachments before test:

Attempting 10 concurrent attachments...

Attachements after test:
(overcloud) [stack@undercloud-0 ~]$ cinder list
+--------------------------------------+--------+------+------+-------------+----------+--------------------------------------+
| ID                                   | Status | Name | Size | Volume Type | Bootable | Attached to                          |
+--------------------------------------+--------+------+------+-------------+----------+--------------------------------------+
| 88820157-a5f2-44ac-acb2-dde0d8ee4fc1 | in-use | -    | 1    | tripleo     | false    | 908ae406-b466-469c-8586-83ecf81101c0 |
+--------------------------------------+--------+------+------+-------------+----------+--------------------------------------+


Attached volume to instance, ran script 
After which still only see a single instance attached to a single volume. 
looks fine to verify.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2020:0712