Cant Register SIP User from Asterisk GUI

Cant Register SIP extension in asterisk 1.6.1 rc3, could some body help me plz ill really appreciate it
asterisk cli gives me the below error
WARNING[23884]: pbx.c:6854 add_pri_lockopt: Unable to register extension ‘o’, priority 1 in ‘default’, already in use

when i try to register a sip extension
thnx in advance

Can you post your users.conf file and default context from extension.conf file

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