[core] Make TDirectory::Append tolerant to identical objects.

When calling TDirectory::Append(obj, replace=true), and obj is already
in the directory, don't issue a warning. Proceed to clear the list of any
duplicates, issuing a warning for each.

This will enable workflows such as:
   auto h = new TH1D(...);
   directory->Append(h, true);

After this change, the above lines work both with implicit object
ownership off and on.

Author: hageboeck

core/base/src/TDirectory.cxx

@@ -207,8 +207,10 @@ void TDirectory::Append(TObject *obj, Bool_t replace /* = kFALSE */)
    if (replace && obj->GetName() && obj->GetName()[0]) {
       TObject *old;
       while (nullptr != (old = GetList()->FindObject(obj->GetName()))) {
-         Warning("Append","Replacing existing %s: %s (Potential memory leak).",
-                 obj->IsA()->GetName(),obj->GetName());
+         if (obj != old) {
+            Warning("Append","Replacing existing %s: %s (Potential memory leak).",
+                  obj->IsA()->GetName(),obj->GetName());
+         }
          ROOT::DirAutoAdd_t func = old->IsA()->GetDirectoryAutoAdd();
          if (func) {
             func(old,nullptr);