Ability to filter out duplicate issues
I was taking advantage of the new filter from #1678 allowing me to quickly enter estimated times we'd omitted from a new project. However, I decided I wasn't going to enter times for issues marked "duplicates X" since we were estimating the time elsewhere in these cases.
As a result, I have issues remaining in my "To Estimate" filter, but I can't tell at a glance which ones I need to make an estimate for and which ones are simply duplicates I'll never estimate. Ideally, I'd be able to exclude duplicate issues from a filter entirely if I so chose.
Granted, if we're going to bother with this at all, might as well allow filtering for "is duplicate" or "is not duplicate" since the extra work involved to support both should be minimal. In my case this would allow both omitting duplicates from the list of things to estimate as well as finding duplicates with estimates that need to be cleared out.
For reference, there's a semi-related issue #1721 which seemed to request the reverse of this issue (namely, the ability to find items "duplicated by X").