The first thing to understand is the terminology that is being used. Duplicate medals are those which are officially provided by the Government when originals have been lost. These are also know as Replacement medals. The duplicate/replacement medal is named like the original but there is a 'D' stamped on the medal to indicate that it is an official duplicate. Some times an 'R' may be stamped on the medal. I've added a couple of photos below as an example.
That being said, there are replica (or reproduction or copy) medals available for purchase. I don't endorse any particular company but examples of replica medals can be seen here. Replica medals should be marked somewhere with the word 'replica' or 'copy'. There are some photos below to illustrate what I mean.