Description of problem:
According to the man page and info page
/usr/bin/id -G should "print only the supplementary groups"

In fact it also prints the primary group

Version-Release number of selected component (if applicable):
Tested on 7.2 & 8.0

How reproducible:
Always

Steps to Reproduce:
1. groupadd testgroup
2. useradd -g users -G testgroup testuser
3. id -Gn
    
Actual results:
users testgroup


Expected results:
testgroup

Additional info:
Could be that the docs are incorrect and listing the primary group is the
correct output?