Appending transformed into truncation

[evanm]

I discovered that, given a script as such:

#!/bin/bash
echo truncate >&1

If it is invoked like so:

echo data>test; ./iobug.sh >>test; cat test

It will truncate the test file, rather than append to it. This is not
behavior I would expect. I'm aware that it's a bit silly to redirect stdout
to stdout, but I recently ran across a script where someone did something
similar, which brought this to my attention.

#!/bin/bash
echo truncate >&2

and echo data>test; ./iobug.sh 2>>test; cat test

works as expected. Can someone explain this?