How do I prevent duplicates, in XSL?

Question

How do I prevent duplicate entries into a list, and then ideally, sort that list? What I\'m doing, is when information at one level is missing, taking the information from

Answer 1

There is no need for an XSLT 2.0 solution for this problem.

Here is an XSLT 1.0 solution, which is more compact than the currently selected XSLT 2.0 solution (35 lines vs. 43 lines):

When this transformation is applied on the originally provided XML document, the correct, wanted result is produced:

   
      156, 154
      Clinic Building
      1947
   
   
      
         156
         3
      
   
   
      
         156
         Contact prints
      
   
   
      
         154
         Negatives
      
   

Update:

I didn't notice the requirement that the container numbers must appear sorted. Now the solution reflects this.