another question about finding duplicates

[Peter Alcibiades]

There's a file which goes roughly:

1	anything
2	anything
3	anything
4	anything

etc

It can always be sorted by the first item.  Sometimes a duplicate entry will 
creep in, so the file will look

1	anything
2	anything
2	anything
3	anything
4	anything

or perhaps

1	anything
2	anything
3	anything
3	anything
4	anything

What's the quickest way of checking for this condition  - when the two items 1 
of any two adjacent lines are identical?  I have found a brute force way to 
do it using filter, but it is really clunky and slow.

Cheers

Peter