In some cases VMFS datastores can have duplicate UUIDs.

Each VMFS datastore created in a LUN has a unique UUID that is stored in the file system superblock. When the LUN is replicated or when a snapshot is made, the resulting LUN copy is identical, byte-for-byte, to the original LUN. As a result, if the original LUN contains a VMFS datastore with UUID X, the LUN copy appears to contain an identical VMFS datastore, or a VMFS datastore copy, with the same UUID X.

ESXi hosts can determine whether a LUN contains the VMFS datastore copy, and either mount the datastore copy with its original UUID or change the UUID to resignature the datastore.

When a LUN contains a VMFS datastore copy, you can mount the datastore with the existing signature or assign a new signature. The vSphere Storage documentation discusses volume resignaturing in detail.