When a storage device contains a VMFS datastore copy, you can mount the datastore with the existing signature or assign a new signature.
Each VMFS datastore created in a storage disk has a unique signature, also called UUID, that is stored in the file system superblock. When the storage disk is replicated or its snapshot is taken on the storage side, the resulting disk copy is identical, byte-for-byte, with the original disk. As a result, if the original storage disk contains a VMFS datastore with UUID X, the disk copy appears to contain an identical VMFS datastore, or a VMFS datastore copy, with exactly the same UUID X.
In addition to LUN snapshotting and replication, the following storage device operations might cause ESXi to mark the existing datastore on the device as a copy of the original datastore:
LUN ID changes
SCSI device type changes, for example, from SCSI-2 to SCSI-3
SPC-2 compliancy enablement
ESXi can detect the VMFS datastore copy and display it in the vSphere Client or the vSphere Web Client. You have an option of mounting the datastore copy with its original UUID or changing the UUID to resignature the datastore.
Whether you chose resignaturing or mounting without resignaturing depends on how the LUNs are masked in the storage environment. If your hosts are able to see both copies of the LUN, then resignaturing is the recommended method. Otherwise, mounting is an option.