Aggregate Comparisons??

I have implemented a login audit on a particular system which catches the users login details, including their application logon name and NT username.

What I want to do is report on users who have logged on to the software using someone else's workstation (i.e. logged on to more than one workstation).

Here's some sample stuff to play with

DECLARE @logins table (
loginName char(20)
, ntUsername char(25)
, loginDate datetime
)

--Insert test data. Please note that loginName and ntUsername are rarely the same
INSERT INTO @logins (loginName, ntUsername, loginDate)
SELECT 'Amy', 'Amy', '20070101' UNION
SELECT 'Amy', 'Amy', '20070102' UNION
SELECT 'Amy', 'Amy', '20070103' UNION
SELECT 'Bob', 'Bob', '20070101' UNION
SELECT 'Bob', 'Bob', '20070102' UNION
SELECT 'Bob', 'Amy', '20070103' UNION --Bob has logged on using 2 different NT accounts
SELECT 'Cal', 'Cal', '20070102' UNION
SELECT 'Cal', 'Amy', '20070102' UNION --So has cal
SELECT 'Dom', 'Dom', '20070102' UNION
SELECT 'Dom', 'Dom', '20070102'

Any ideas? I just can't think of the logic needed to get what I want.

Any extra info needed - just ask!
Cheers