ConceptService getConceptsByMapping should only return distinct concepts

Currently, if a Concept has 2 mapping terms to the same Concept Source, which have the same text but differ in casing, the ConceptService.getConceptsByMapping method will return a List of size 2, with the same concept returned twice.  This should be fixed such that only distinct concepts are returned.

I noticed this due to some things that started failing in my 2.5.x environment after this was backported here:

https://github.com/openmrs/openmrs-core/commit/bcfcaf99aeee0cf1b5c4ce7a793e92a0c97ed70a