[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.

Author: guitargeek

roofit/histfactory/src/HistoToWorkspaceFactoryFast.cxx

@@ -71,6 +71,7 @@
 #include <fstream>
 #include <iomanip>
 #include <memory>
+#include <set>
 #include <utility>
 
 constexpr double alphaLow = -5.0;
@@ -1478,6 +1479,11 @@ 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