I had a similar issue like this mounting our NFS storage . This is what I did to resolve the issue and I'm hoping it'll lead you to resolving your specific issue if it's not exactly like mine:
The datastore name it's complaining about that already exist "does" in fact exists. What you're trying to do is tell the ESXi host to use that existing datastore that is already in inventory instead of creating a new datastore inventory with essentially a new name. On the new host you're trying to mount an already existing datastore to, you must make sure "everything" is exact, especially the "folder and Datastore name" part. When I say exact, I'm talking even the forward slash "/"
My issues was this:
When I mounted the datastore for the first time I configured it as shown:
Server: 10.10.0.10
Folder: /vol/datastore01/
Datastore Name: netapp_control_a_sql
Now when I try to do this on another ESXi host, I had it configured like this:
Server: 10.10.0.10
Folder: /vol/datastore01
Datastore Name: netapp_control_a_sql
All is gravy right? until vCenter appends (1) to the datastore name and inventories this as a different datastore. What do you see wrong there? the issue here is the ending forward slash "/" in the folder field that was included on the initial config and the additional config was missing it. I included the missing forward slash and the problem was resolved. I can imagine this being an issue with caps or anything differentiating the initial mount and the referencing mount.
-Sonny