Detecting duplicated components

Add check to detect duplicated components of a record
when parsing a TRLC file.

Example:
```
Something duplicated
{
	description = "This is fine!"
	description = "This is the duplicate."
}
```

The behavior of the `Record_Object` class is changed such that the `assign` method only
assigns components to the record if it has not been assigned already.
This is implemented by checking if the field type is different from `Implicit_Null`.

The `Parser` class asks the `Record_Object` if the component is `Implicit_Null`.
If no, then an error is sent to the message handler.

trlc/ast.py

@@ -2998,6 +2998,9 @@ def to_python_dict(self):
         """
         return {name: value.to_python_object()
                 for name, value in self.field.items()}
+
+    def is_component_implicit_null(self, component) -> bool:
+        return not isinstance(self.field[component.name], Implicit_Null)
 
     def assign(self, component, value):
         assert isinstance(component, Composite_Component)
@@ -3008,6 +3011,8 @@ def assign(self, component, value):
                                   Implicit_Null,
                                   Unary_Expression)), \
                 "value is %s" % value.__class__.__name__
+        if self.is_component_implicit_null(component):
+            raise KeyError(f"Component {component.name} already assigned to {self.n_typ.name} {self.name}!")
         self.field[component.name] = value
 
     def dump(self, indent=0):  # pragma: no cover

trlc/parser.py

@@ -1810,6 +1810,11 @@ def parse_record_object_declaration(self):
             comp = r_typ.components.lookup(self.mh,
                                            self.ct,
                                            ast.Composite_Component)
+            if obj.is_component_implicit_null(comp):
+                self.mh.error(self.ct.location,
+                              "component '%s' already assigned at line %i" %
+                              (comp.name,
+                              obj.field[comp.name].location.line_no))
             comp.set_ast_link(self.ct)
             if r_typ.is_frozen(comp):
                 self.mh.error(self.ct.location,