When we build the search paths for the JSON ICDs it might happen to have the same directory multiple times. For example if we have "XDG_CONFIG_DIRS=/usr/share" and "XDG_DATA_DIRS=/usr/share" we will look in "/usr/share" two times and duplicate all the JSONs that we find.
Usually this is not an issue but with some games, e.g. Red Dead Redemption 2, this can cause a crash at startup https://github.com/ValveSoftware/steam-runtime/issues/336
With this commit we filter out the directories that we already visited to prevent this kind of issues.
Fixes: #48
/cc @smcv