You can create a hyperlink from one case to another very easily: just type the word "case" followed by the number of the case to link to.
For the purposes of this description, let's say you're entering comments in case 10. If you type "see case 20", FogBugz will turn "case 20" into a link to case 20. (If you like, you can use the word "bug" instead of "case").
Not only does this create a link to case 20 within case 10, but when you view case 20, you will see case 10 listed as a "Related Case". (This, of course, is a clickable link back to case 10.)
When one bug report is a duplicate of another bug report, you should resolve it as "Resolved (Duplicate)". That lets you enter the number of the bug that it is a duplicate of, and this becomes a hyperlink. Once again, this link goes both ways between the cases.
