usermod -G does not take effect immediately

I'm having trouble with the following behavior on
Gnu Linux:

  used 'usermod -G users,demo3 garg' to
add user 'garg' to the users and demo3 groups.

 Then, when I do 'groups' as user 'garg', I get
only 'users'.

 If I do 'groups garg' as user 'garg', then I get
'users demo3'.

 Can someone tell me what I'm doing wrong? This is
causing a problem as user 'garg' is not being able
to access some files properly. Is this some sort of
a caching problem?

 Thanks for any pointers.

Pankaj

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