The error is because there were 2 controllers returned (the same one twice).
Could it be that you have 2 vSphere connections open ? Do a
$global:defaultviservers
If there is more than 1 entry displayed, you should check if you are running in multi-mode. Do a
Get-PowerCLIConfiguration
Does 1 of the scopes say "multiple" ?
If yes, disconnect 1 of the vSphere connections. For example by doing
Disconnect-ViServer -Server $global:defaultviservers[0] -Confirm:$false
Then check if Get-ScsiController only returns 1 object.