I noticed that I am upload the same file multiple times. I think XF should show some kind of message informing the user that they've uploaded the same file multiple times and disallow them. Seems like a waste of space to allow them to upload the same file multiple times to the same message.
Well, the thing is you can't easily reuse it atm can you?
Images and other media really need to be handled well from the start. Each member should have ready access to all images and other media they have uploaded so that they can upload, sort and reuse files quickly and easily. Having images that are often very useful content buried in a one-off attachement is a waste. You should be able to drop any image/file you already have uploaded directly into your posts/threads etc.
We get a lot of google images traffic from our galleries because they are categorized, searchable and have tons of useful user contributed images. BNone of that happens when the images are buried as attachments. Not to mention the wasted bandwidth and storage when members re-upload the same file again and again.