One of my students has reported the same error - it seems that the Project needs to be on the large side, especially having many Assets, many Blocks. The next release might reduce this problem because it loads Blocks faster but clearly there is a bug.
Mine has very few blocks in one screen, but "many" assets(25). It also happens on the testserver.
@Peter You are correct, I just uploaded 2 AIAs with same name. And it uploaded successfully.
Your Internet is fast 
. Try using any bigger file. BTW, You tried to refresh the page ? When I upload 2 AIAs with same name. It shows only one but when I refreshed the page. I found 2 Projects with same name.
@Peter You are a epic Bug-Hunter 
I was also able to do this, albeit this time it was two duplicates; the .aia was 2.2MB in size:
Edit: This was performed on (Chromium-based) Microsoft Edge.
I tried, but I can't do that, even on the old one (I used Chrome).
I don't know why, that's strange...
It probably depends on the specs of your computer. I, for instance, usually only have 400MB of free RAM