Why do some universities refuse to issue duplicate diplomas? Many universities have a policy of not issuing a duplicate diploma for a degree, unless your original copy is incorrect, damaged, or lost.

One example:


  University Policy forbids the issuance of multiple copies of a diploma. Each diploma is considered a one-of-a-kind legal document.
  http://www.queensu.ca/registrar/diploma-replacements


Another example:


  Replacement Graduation Diploma
  
  
  A graduation diploma will be replaced in the following cases:
  
  
  When a diploma has been lost or damaged.
  When the name of the holder has been changed under the regulations for the change of name process as per University records.
  
  Return of the original diploma with the request is required if the reason for replacement is due to damage or an official name change.
  In cases of lost or destroyed diplomas, the holder must submit a Statutory Declaration (form available below) to the University Registrar and have it signed and sealed by a Notary Public*, legally swearing that the original diploma has been lost or destroyed. The condition of the reissuance of a diploma in cases of loss or destruction is that if the original should be found at any time, the replacement diploma must be returned to the Registrar’s Office at Carleton University for cancellation.  
  
  
  http://carleton.ca/registrar/academic-evaluation/replacement-diploma/


The most explicit example:


  Stanford University will not issue duplicate diplomas under any circumstances. This measure is taken to protect Alumni from identity theft. If your diploma was permanently lost or destroyed, a replacement diploma may be ordered from the Office of the University Registrar. You must fill out the Replacement Diploma Form, have it signed by a Notary Public ...
  
  (PDF) The Stanford diploma is a unique document; neither copies nor duplicates are available.
  https://registrar.stanford.edu/students/diplomas/replacement-diplomas




But then, there are other universities that have no, such, restrictions on duplicate diplomas.

The inconsistent policies between universities seem to suggest that some see it as a problem and some do not. Why would duplicate diplomas be a problem in practice?

My speculation, with no evidence provided:


Giving a duplicate diploma to another person who coincidentally has the same name as you?
This rule was born in an era before telecommunications and online services? You physically carried and showed your diploma to prospective employers and academic admissions officers?
It comes down to what the theory is about what a diploma is.  Some consider it a piece of paper that you can hang up and not an explicit representation, some consider it a legal document.  (Having degrees and working in Higher Ed for 20 years, often with various policies including those of Registrar's offices gives me some insight.)

Reasons for the policy: 
These policies are based on legal theory or precedence regarding identify theft and legal documentation.  Schools with a higher 'reputation' or a better 'ranking' will have more restrictive policies to protect themselves. 

Queens U, above, states "Each diploma is considered a one-of-a-kind legal document."  

If you give/sell a copy to someone to attempt to represent themselves as having a diploma, then it might be fraud. 

What is the difference between having two originals, and simply saying 'I lost my original, can I buy another?'  --  It could be quantity. If you can go to a web site, pay $40 each and buy 100, then sell them all on Ebay with a "custom name" for $500, that's fraud.  The schools are attempting to protect themselves in a few ways. 
Here's a sample of a fake diploma for sale:
http://washingtondc.craigslist.org/nva/fns/5668595297.html 

Here's a list of real diplomas for sale:
http://www.ebay.com/sch/i.html?_from=R40&amp;_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xdiploma+degree.TRS0&amp;_nkw=diploma+degree&amp;_sacat=0 

People lie on resume's and they lie with diplomas.  

Reasons why such restrictive policies are nonsense: 
IANAL, though, legal documents often have multiple copies, both original and duplicate.  Ever buy a house?  It is normal to have to sign multiple 'originals' of the contract documents.  

What is the difference between having two originals, and simply saying 'I lost my original, can I buy another?' regardless of whether the original is lost.  

If you have a diploma, and you make a photocopy, frame it, and hang it in your office, and keep the original in a fire-safe, seems ok to me. 

For degree verification, I've been asked to provide official, sealed, stamped transcripts, not a copy of the diploma. 

In summary - the policies will vary based on laws, precedence, legal theory, and/or policy.