How to handle multiple (same) request in node.js?

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MySQL

Node.js

I have written a function to insert record in 'user' table (mysql db). This user table has only 'id' primary field with auto increment and no any other field is unique, but In program I am checking if value of a field (let say for example the field is 'username') is exist then don't insert record. But the I am getting duplication record because the Two requests are coming to the server at same time (I am also saving the creation time in (creation_date) field). When I am manually inserting the record one by one then It Works perfect but using program or JMeter testing tool firing the multiple request at the same time then duplicate record found in table.

I can make username unique field but in my situation unique key length exceeds the key length.

Any other Solution please suggest.