Now Ill preface this with mentioning that I do master writs en masse. I will write down the writ requirements and head to my guild hall and craft 30-40 writs at once. Then I will head to Rolis, take a psijic and turn each writ in one by one.
I had a bunch of clothing writs on my char, and in my bank. Some were duplicates where I had 1 on char and 1 banked. So I withdrew all my writs from the bank and went to town. I made note of the writs that I had multiple of, because I have to craft extra of those particular items.
I went through my process as normal, except now I have no more writs, and all the extra items in my inventory.
In other words, the writs I had multiples of seemed to have been deleted when turning in the first one.