Double checking validation PHP and Ajax -

currently i'm using ajax check register validation, feel if internet connection not enough, ajax not working , makes users register same username. how can double check register validation avoid duplicate usernames in database?

if field in database set unique won't problem since error returned i.e.

create table user (     username varchar(20) unique not null,     ... ) 

when insert run on table potential username taken, error kicked back.

should mention while client-side validation , being fancy on webpage pop-up divs , highlighted spans, validation should rechecked on server-side.