When trying to rename the tabs and labels for the custom Contract object in online CRM, the following error appears:
Error: You cannot reuse a standard salesforce.com name.
There is already an existing standard Salesforce object with the same name. The name cannot be used even if the original Salesforce object is renamed.
There is no solution or workaround for this occurrence. The custom tab has to be renamed another way.
Salesforce Documentation: Giving custom objects the same name as standard objects is NOT RECOMMENDED