While adding a user the other day, I noticed that if somebody else
started another adduser script before I finished adding the current
user, both would get assigned the same UID.  Now, technically, this is
not a bug, since the script asks you which UID you want to use,
however, it would be quite hard to guess if somebody else is adding a
user and what UID they're using.

Fix: 

My suggestion would be to lock the password file from the beginning
of the script and notify the user if it can't get a lock.  That way
there can only be one adduser script running at any given time.

Looking at the script, however, this does not seem like a good idea.
It uses a subroutine called 'append_file' that takes care of the
locking while appending a line to a file.  Personally, I'm going to
do the pw file lock at the beginning, and make a modified append_file
routine specifically for the password file (which will already be
locked).  Hopefully, the author will come up with a 'cleaner'
solution later on :)
How-To-Repeat: Run the adduser script and start answering the questions.  Try adding
a dumb user (test1) and get to the point where it asks for the UID.
When you get there, start an adduser script from another session and
add another dumb user (test2).  When you get to the UID prompt, you
will get the same UID for this user.  If you go through and add the
users, the script will happily add the two users with the same UID.