It has never happend to me before. Have you tried rescanning all storage devices?
I found this VMware KB that maybe explains your problem: VMware Knowledge Base
"The snapshot LUNs issue occurs when the ESXi/ESX host does not confirm the identity of the LUN with what it expects to see in the VMFS metadata. This issue occurs after replacing SAN hardware, firmware upgrades, SAN replication, DR tests, and some HBA firmware upgrades. This issue also occurs in some ESXi/ESX host upgrades from 3.5 to 4.x (due to the change in naming convention from
naa). However, this is a rare occurrence. For more/related information, see the Managing Duplicate VMFS Datastores in the ESX Configuration Guide for ESXi/ESX 4.x and vSphere Storage Guide for ESXi 5.x and 6.x."
When one ESXi/ESX host force mounts a VMFS datastore residing on a LUN is detected as a snapshot, an object is added to the datacenter grouping in the vCenter Server database to represent that datastore.
When a second ESXi/ESX host attempts to do the same operation on the same VMFS datastore, the operation fails because an object already exists within the same datacenter grouping in the vCenter Server database.
Because an object already exists, vCenter Server does not allow mounting the datastore on any other ESXi/ESX host residing in that same datacenter.
Hope this fit your issue