I can't match the screen shot to your written description of the design of your database.
Allow creation of records via this relationship dose not automatically create records in the related table. It merely makes it possible to do so from the context of the other table via a portal or a few other methods--such as entering data into a field from the table where "allow..." was enabled when on a layout based on the related table occurrence.
So if you are creating a new record and data pops up in fields from a related table, this sounds like a potentially very serious issue with your data. It sounds like your new record already matches to existing records in the related table and this should not be the case.
Hey, thanks for helping. I did now solve the problem. Serial number in person table is formatted as a text and in aputaulu too.
And there was existing keys which where higher than next increment value. Problem is now fixed, formatting text to number, validates that the key is unique. Replaced old data with new unique keys.