recover - Entry "cn=schema" single-valued attribute "nsSchemaCSN" has multiple values - how?

[lejeczek]

Hi guys.

One of IPA masters somehow got itself into trouble:

[25/Nov/2020:10:42:51.918481532 +0000] - ERR - 
slapi_entry_schema_check_ext - Entry "cn=schema" 
single-valued attribute "nsSchemaCSN" has multiple values
[25/Nov/2020:10:42:51.924870263 +0000] - ERR - 
setup_internal_backends - Please edit the file to correct 
the reported problems and then restart the server.

It would be nice to know how such a case can occur but most 
important question is - how to fix it?
Any suggestions I'll greatly appreciate.
many thanks, L.