I don't know of any way to merge existing issues. But if you've converted one issue to a project and want to resolve another issue with that same project, you can go to the issue's Overview and set the resolving project to that existing project.
You have to do this from the issue side, not from the project. But we've found it handy lately as we have users submitting requests at an earlier stage and sometimes get a couple people submitting the same thing.