I see that the congressperson was right because most inputs require the a first name in other to process.
If not, you can't send anything for the process to be completed.
In most of these cases, The last name is often used in place of the name.
But in government forms filings, this should always reflect what is on your birth certificate.
So using the first name the same as last name can be okay for inputs. This situation will definitely be resolved with citizenship because you are allowed to add or change names.