Already tried to work around it naively by using a function that
would retrieve the key, which didn't seem to work but is that because
it just is not allowed to use a function call in the group-by or
because I made a mistake in my select statement.

presumably the latter but I don't think you've give enough information
to give much help. In particular does "didn't seem to work" mean
generate an error (if your first choice above were true you would get a
static syntax error on the function call) or not generate an error but
generate wrong output or no output or ....

Can't you post a small (dozen line) input file with a couple of
duplicated uris and say what files you want generating. It could be
_anything_ like for instance
document(concat($sourceFolder, @FILENAME))/maximo:*/maximo:*/maximo:*"
not selecting anything because you have the path wrong, but how can we
guess? If you make a _small_ self contained example that people can run
and which shows the issue then it is probably easy to fix.

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