UPDATE all values of a column with values of a related table
Run the following select in order to find cases where you have more then one record for the key you are using. Once you find the records with duplicate values you can figure out how to deal with them and it could be removing them from the other table or updating the select to exclude them.

SELECT GH220_INVENTAR_NR, 
       COUNT(0) 
FROM GH220_AUSSTATTUNG 
GROUP BY GH220_INVENTAR_NR 
HAVING COUNT(0) &gt; 1