compare within a list

Hi,

I have a list of elements with ids as:

set elementlist { {100 101} {1000 1001} {101 100} {1001 1000} }

I need to check for duplicate ids within the list. whichever item is existing twice should be written as output.

expected output:

{100 101} {1000 1001}

or

{101 100} {1001 1000}

Can anyone help me with this? 

Thanks in advance!