My question from earlier today, "I did want to" versus "I wanted to", was labeled as a duplicate. I can live with that. However, it appears that my reputation took a two-point hit. This I don't understand. I made a good-faith effort to search for the answer first and didn't find the specific one referenced (did shoot vs shot), so I asked my question. It wasn't a "dumb" question (there are some) and I believe it was polite and well written, so I'm don't understand why there is a penalty. Surely one isn't expected to read every question on the site before asking a new one, just-in-case. My question had one up-vote, and the one answer had four up-votes before it was shut down. This does not seem to be in the spirit of encouraging participation.
Questions asked must show some sort of effort or research. The first thing you must do before asking a question on any stack exchange site is searching through the specific site for a response. This is expected of any good user. Researching outside sources and including one's findings is something expected of excellent users. Answers to questions asked on Stack Exchange sites should not be able to be found in the first page of a google search. Here you can find helpful hints on how to search the Stack Exchange sites.
If a question is a duplicate, it very often means it was poorly researched. Most users will assume you have not done your homework. The quickness with which a question is marked as a duplicate can also be a likely indicator of the same.
If you did your research and found similar questions that don't quite provide the answer you are looking for, you should include them in the new question you are asking and explain how they are different from your own.
These are EL&U's gudelines for asking questions.
Hovering over the downvote button displays the following tooltip:
This question does not show any research effort; it is unclear or not useful.
I did read your question at the time. Although I didn't actually downvote it, I might have because your question did "not show any research effort". The only additional information in your question was that (paraphrasing) you often changed your mind about which to use. That gave me the impression of no research: here's the issue; I can't decide which to use when; please help!
As others have said, only the downvoters can explain why, but I would also say, do not assume that they are because it was considered a duplicate: they may have been for other reasons.
Is there a way it can be made compulsory to comment for all downvotes? This would be constructive, not only for the answer but also for the whole site. It will also encourage the voter to think and serve as a check on cowardliness.