Warrior/Rogue - Combines the stun locking and tanking abilities of warrior with the high damage of rogues. Yes it is a very expensive build and very hard to master so not really recommended for you if you are starting new.
Warrior/Cleric - The 2nd most popular build to be found in sro which combines tanking, good damage and high survivability. It can use 1,2 or 3 weapons at once providing you with a lot of room to mix and match your skills.
Warlock/Cleric - Arguably the fastest pve grinder in sro and an almost OP build. Now a little weakened by the loss of trap/sleep bug it is still a very potent build in pvp.Although yes holy word/spell does leave nearly all debuffs ineffective and leave only a single possible combo for victory in case of pure str builds. It is still however a very good build that will not disappoint in pvp. It does however have a lot of skills, requires a lot of farming and does have a bit of a learning curve especially for pvp. Please remember that this class' strength lies in its ability to debuff/disable and dish out high dps. Not in burst damage like a wizards nukes or a warrior or rogue's crits.
Rogue/Cleric - An underestimated char, it works wonders in groups and is a very good build over all. At later levels with a decent set it has some very nice survivability in 1v1 pvp(see some videos of good pvpers to see what i mean) after bladers, warriors and glaviers. Please keep in mind that like most mmorpg's SRO's rogue is a 1v1 specialist both in pvp and pve. It has no area skills but with stealth and other skills it can effectively play the role of an assassin using either xbow(sniper) or daggers(ninja).
Finally, A build that is good in group pvp and duelling is a criteria that is easily fulfilled by any of these chars when played well. However, unique hunting is a different matter. Try to understand that with the prevalent childish mentality in the minds of sro's players it is very difficult for 1 to actually find and kill a unique when it spawns, even if you do manage to find it b4 it dies you will have a lot of competition as several ppl will be trying to kill it(mostly wizards and rogues).
So Rogue/Cleric would probably fulfill your needs best given group pvp, duelling, caves and unique hunting. If you feel like taking another build, please do so. I think I've summarized all the aforementioned builds decently in this post
)
... just kidding, rogue/cleric it shall be. 
