This is a great question! The answer is yes to both. Let me explain...
It is a common practice to develop a talent database from casting jobs. This can save time searching for talent each time around, and enables notes and feedback to be carried forward as desired. While I don't know of a casting director that deletes submissions after a project, I cannot speak to the percentage that take the time to dig through these old auditions. I can tell you that I've gone into my own casting archives before to see if there is anyone I want to request a submission from that maybe didn't see my current casting call. So it definitely happens.
That said, it is always necessary to send a new and updated submission each time. Not only do different projects have different requirements (even from the same CD), but it shows a measure of availability/interest from the actor for that particular film. An equally important reason to have an new submission from prospective talent is to have current materials (headshot, resume, etc.) that reflect any changes in their look and additional credits/skills/training that may have been acquired.
Thanks for reaching out! Keeping knocking on doors, and be ready to smile when someone opens it.
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