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Typically, you wouldn't have BOTH mob number and surname in the both tables. You don't want situations where if a value changes (like surname), it needs to be changed in both tables, and you typically do not want key fields that you join on to be fields in which values can change. So I would recommend just keeping surname in the "people details" table, and remove it from the other.
To further to that extent, you typically do not want to store the other information in multiple tables - unless the information may change and you are trying to track it for historical purposes. Typically, you would just join the two fields in a query, and return all the fields that you want to see in the query (so if there is no need to track what the data was at an exact point in time, there is no need to store it in the other table also).