summarycombine: Only save one copy of PSD and calibration envelope files when they're the same for multiple runs

When multiple runs are combined with summarycombine that have the same PSD and calibration envelope files, these files are currently duplicated--see, e.g., the calibration files for one of the O3a IMR consistency test runs. While these files are not very large, it still seems preferable to only store a single copy, if possible.