BUG updateCMSFields creates duplicate tabs in some cases

[muskie9]

Attempting to update the MemberProfilePage::getCMSFields() resulting FieldList via a data extension can cause duplicate tabs. For example: attempting to add an additional GridField to a new tab under Root.Profile will cause the Profile tab to duplicate.

A solution of implementing the page's fields in beforeUpdateCMSFields() appears to resolve the issue as all tabs and fields are built prior to any extension calls.