avoiding duplicate solutions

I am looking for background information on
avoiding duplicate solutions, what causes them,
etc.

I am using the model found in lib/propria/zebra.pl
to generate a solution to a puzzle.  The solution is a list
taking the form [u(number,name,color), u(number,name,color)...]
where number and name are unique but color is not
(20 names, 20 numbers, 8 colors).

I get the correct answer, but More is indicated. 
Clicking More repeatedly gives the same answer as before. 

Code to puzzle (about 300 lines) available on request.

Doug Edmunds
22 March 2000
edmunds@pacifier.com