The cast list

It is a maybe stupid question about the cast list here on IMDB.

Why are JLH, Sigourney Weaver and a couple others credited for playing multiple characters. The list also mentions the fake names, the characters use.
I find that weird.
They are not different characters, it is one character using a fake name, a fake accent and a disguise.
I have seen it with other titles too.

To take a well known example of what, I find, is actually one actor playing multiple characters, Eddie Murphy and Arsenio Hall in Coming to America.

One could argue, that it is justifiable to credit Robin Williams for both Daniel and Mrs Doubtfire, but in the case of Heartbreakers, I see no reason to credit JLH for Wendy.