The gold badge dupe hammer is a little scary at first, but you soon get used to it. ;) (Obviously, I don't yet have close-voting privileges on Physics, but on SO I have a gold badge in Python).
If you're pretty sure that the question is a duplicate, but you aren't confident that the question you nominated is the best dupe target, just close it anyway. The dupe target list can have multiple entries, and it can be edited after the question is closed.
The point is to get dupes closed quickly. Dupe questions aren't inherently bad (unless it's a common question and the OP did zero research to find an existing question), but we do want to prevent the proliferation of dupe answers. I'm not suggesting that questions should be dupe-closed against targets that are only vaguely related. But if you've found a target question that is a reasonable match to the new question, then close it quickly. And then do a more in-depth search for a better target(s).
It may also be worthwhile writing a brief comment to the OP, asking if the dupe target satisfies them. A dupe target that contains the required info isn't much use if it's too complicated for the OP to understand, or too simple & general to address their specific concerns. Also, we don't want people to think that they're being punished by having their question dupe-closed, that's less likely to occur if we ask them for feedback regarding the dupe target.
If the OP doesn't like the target, try to persuade them to clarify the question as to why your dupe target isn't appropriate. Of course, many OPs, especially new contributors, will just reply in comments, and not comprehend that such info actually belongs in the question body. In such cases, it may be simpler to just edit in the info yourself than to teach them how Stack Exchange sites work. ;)
Sometimes, the dupe target is what the OP really needs, but they just need a helping hand to see that. And sometimes that can be accomplished with a sentence or 2 and a relevant link in a comment. (Other times it may be necessary to write a brief actual answer to the new question that bridges the gap between the OP's skill level and the info at the dupe target, especially if the answers at the target assume advanced knowledge, or if there isn't clear agreement between them. True, such a bridging answer could be posted on the dupe target page, but if it's a big page the new answer is likely to get buried. OTOH, answering & dupe-hammering the same question is somewhat controversial. ;) )
If you want to nominate a dupe target, but really don't feel comfortable closing the question single-handedly, there's a simple solution: just post a manual "Possible duplicate of X" comment. That makes the link available to other close-voters. And then re-visit the question a few hours later to see if it's gathered some close votes. If it has, then feel free to hammer it.
Another option with a borderline question is to mention it in the chat room, to see if other people agree with your target, or if they have better suggestions.