You should still check for previous reports, but if there is any doubt at all that your bug is different you should file it separately. For example, two crashes are only the same bug if the backtraces are identical. If there is not enough information to determine whether the bugs are the same, it's better to file a duplicate than to risk having to disentangle multiple bugs out of one report later.
(There's some disagreement among the development team about this, so I can only really claim that the above applies to installer bugs. However, if you make a good-faith attempt to search for duplicate bugs, and note in any new bug reports that you think certain other bugs may be related, then I don't think too many people will complain.)
You should still check for previous reports, but if there is any doubt at all that your bug is different you should file it separately. For example, two crashes are only the same bug if the backtraces are identical. If there is not enough information to determine whether the bugs are the same, it's better to file a duplicate than to risk having to disentangle multiple bugs out of one report later.
(There's some disagreement among the development team about this, so I can only really claim that the above applies to installer bugs. However, if you make a good-faith attempt to search for duplicate bugs, and note in any new bug reports that you think certain other bugs may be related, then I don't think too many people will complain.)