[RF][HF] Error out in HistFactory in case of duplicate channel names

Duplicate channel names are not allowed, because it would result in name
collisions for the channel pdfs.

(cherry picked from commit 08dc832)

Author: guitargeek

roofit/histfactory/src/HistoToWorkspaceFactoryFast.cxx

@@ -67,6 +67,7 @@
 #include <fstream>
 #include <iomanip>
 #include <memory>
+#include <set>
 #include <utility>
 
 constexpr double alphaLow = -5.0;
@@ -1474,6 +1475,11 @@ RooArgList HistoToWorkspaceFactoryFast::createObservables(const TH1 *hist, RooWo
         Error("MakeCombinedModel","Input vector of workspace has an invalid size - return a nullptr");
         return nullptr;
      }
+     std::set<std::string> ch_names_set{ch_names.begin(), ch_names.end()};
+     if (ch_names.size() != ch_names_set.size()) {
+        Error("MakeCombinedModel", "Input vector of channel names has duplicate names - return a nullptr");
+        return nullptr;
+     }
 
     //
     /// These things were used for debugging. Maybe useful in the future