Hello Hemant,
vCenter comprises of SSO, Web Client, Inventory Service. vCenter. While upgrading to 5.1 U3, it's best to upgrade all these components, so you can be on a n even release.
Now, for which component is it giving already upgraded message?
Your upgrade goes in the order SSO > WC > IS > vCenter
Latest release in 5.1 Chain:
VMware vSphere 5: Private Cloud Computing, Server and Data Center Virtualization
Suhas
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Don't Backup. Go Forward!
Rubrik