Retain property ids when they are already set and unique

This will enable setting loop unwinding assertion ids in GOTO programs
by goto-instrument's loop unwinder.

Author: tautschnig

src/goto-programs/set_properties.cpp

@@ -59,6 +59,15 @@ void label_properties(
     if(!it->is_assert())
       continue;
 
+    if(!it->source_location().get_property_id().empty())
+    {
+      if(property_counters.emplace(it->source_location().get_property_id(), 0)
+           .second)
+      {
+        continue;
+      }
+    }
+
     irep_idt function = it->source_location().get_function();
 
     std::string prefix=id2string(function);