Pre-requisite for standby DB to be part of DG, it should have same dbid.
Refer: http://docs.oracle.com/cd/B19306_01/server.102/b14239/standby.htm#i58150 for additional information.
If your Database A and Database B as same dbid, then you can configure DG between these two db's.
If dbid is not same, then above database CANNOT be part of Dataguard.
To create physical standby you can use RMAN utility for easier setup.
Refer below link for assistance
I have a server B which is a clone of A hence both contains the same oracle Database. (10.2.0.4)
This morning I made changes in server A which is not yet in server B. Now I want to configure Dataguard making server A the pry DB.
Will I have to duplicate database A on B afresh or I can continue.
Also, if I have to duplicate A afresh will it overwrite the database on B.
Please, can someone advise.
You cannot configure Dataguard on server B when you performed clone from A.
For the clarification can you tell what method you used to clone? Also you need to check RESETLOGS_CHANGE$# from both databases of A & B.
If the resetlogs change is differ, then you need to perform fresh refresh from A. The refresh method you can ues either RMAN restore or duplciate or any method which is comfortable to you.
You will have to recreate the database as standby at server B from the backups of primary database at server A.
If you have already done duplication part you can move on the archive log shipping and apply the archive logs on the server B which I suppose will be the physical standby.