It would be great to have some sort of an ajax-populated region that appear when you start typing in an issue Summary or Description, which shows similar issues.

I think this would help reduce the number of duplicate issues in large projects as often two separate users create the same issue (or one forgetful one makes two!)

The best implementation I've seen of this is at StackOverflow when you create a question. As you type in the subject it's performing some sort of a SQL full-text comparison of what you typed to the Question table. They also perform this when you enter the actual body of your question (analogous to JIRA's Description field). See the screenshot for an idea, or actually go try it out at http://stackoverflow.com/questions/ask