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If you feel comfortable proceeding without an agent (i.e. you've done this dance before), letting the seller's agent "double dip" is a great way to get your offer accepted if there is competition. If there isn't competition, it can be a decent way to get a discount. Additionally, they may be more likely to keep you informed on how to make your offer competitive.
If you haven't purchased properties before, or aren't comfortable with the process, it's probably worth getting your own agent.