Commiting unchanged tree

I thought that calling

git commit --amend -m "A message"

twice in a row should lead to an error saying "nothing to commit (working 
directory clean)", but it doesn't. Or at least, the result of the second commit 
should be the same as the result of the first one, but I keep getting a 
different hash each time

# git commit --amend -m "A message"
[master b8cf1c2] A message
# git commit --amend -m "A message"
[master 6c8ea30] A message
# git commit --amend -m "A message"
[master be318b3] A message

I wonder if everything is fine with my repo since I was doing some funny 
rebasing and filtering and I'm quite new to git.