VirtualHost in Apache

Just when I thought I was competent about various settings of virtual host in Apache, I encountered this error when adding a new web site :

"Starting httpd: [Wed Jan 10 19:43:20 2007] [warn] VirtualHost http://www.bya.org.hk/ overlaps with VirtualHost http://www.amrita.org.hk/, the first has precedence, perhaps you need a NameVirtualHost directive"

The result was that access to http://www.amrita.org.hk pointed to www.bya.org.hk.

I had to add the NameVirtualHost 202.81.252.116:80 in "httpd.conf" to make things work again.