it will be named after a Mr Berggren which means that the correct
spelling is Carex berggrenii (double -ii).

Iris Cohen schreef
That is the way it is spelled in the trade.

The spelling "berggreni" would only be acceptable if Berggren is a
mountain, an island or a lake, or something like that.

I agree. It may be a typo in IPNI or IK. A Google and atlas search
revealed that there are lots of people named Berggren, but no place by that
name.

+ + +
In the IK/IPNI such things usually are not typo's but the original spelling
(as it was published way back then). In those days latinization of personal
names had not yet crystalized: the spelling "berggreni" was what Petrie felt
it should look like. The present ICBN disagrees.
PvR