Simplify conflict shape loading conflict handling

There was a check in the loader visitor around duplicate members that is
no longer necessary now that duplicate members are detected while
loading aggregate shapes. Beyond the removal of that if statement, this
change improves the readability of the conflict handling code and gives
more context as to why some conflicts are allowed.

Author: mtdowling

smithy-model/src/main/java/software/amazon/smithy/model/loader/LoaderVisitor.java

@@ -238,30 +238,27 @@ private void onTraitDefinition(ShapeId target, TraitDefinition definition) {
     private boolean validateOnShape(ShapeId id, FromSourceLocation source) {
         if (!hasDefinedShape(id)) {
             return true;
+        } else if (Prelude.isPreludeShape(id)) {
+            // Ignore prelude shape conflicts since it's such a common case of
+            // passing an already built model into a ModelAssembler.
+            return false;
         }
 
-        // Ignore prelude shape conflicts since it's such a common case of
-        // passing an already built model into a ModelAssembler.
-        if (!Prelude.isPreludeShape(id)) {
-            // The shape has been duplicated, so get the previously defined pending shape or built shape.
-            SourceLocation previous = Optional.<FromSourceLocation>ofNullable(pendingShapes.get(id))
-                    .orElseGet(() -> builtShapes.get(id)).getSourceLocation();
-            // Cannot ignore duplicate member definitions.
-            boolean canIgnore = !id.getMember().isPresent()
-                                // Ignore duplicate shapes defined in the same file.
-                                && previous != SourceLocation.NONE
-                                && previous.equals(source.getSourceLocation());
-            if (canIgnore) {
-                LOGGER.warning(() -> "Ignoring duplicate shape definition defined in the same file: "
-                                     + id + " defined at " + source.getSourceLocation());
-            } else {
-                String message = String.format("Duplicate shape definition for `%s` found at `%s` and `%s`",
-                                               id, previous.getSourceLocation(), source.getSourceLocation());
-                throw new SourceException(message, source);
-            }
-        }
+        // The shape has been duplicated, so get the previously defined pending shape or built shape.
+        SourceLocation previous = Optional.<FromSourceLocation>ofNullable(pendingShapes.get(id))
+                .orElseGet(() -> builtShapes.get(id)).getSourceLocation();
 
-        return false;
+        // Ignore duplicate shapes defined in the same file (this can happen
+        // when the same file is included multiple times in a model assembler).
+        if (previous != SourceLocation.NONE && previous.equals(source.getSourceLocation())) {
+            LOGGER.warning(() -> "Ignoring duplicate shape definition defined in the same file: "
+                                 + id + " defined at " + source.getSourceLocation());
+            return false;
+        } else {
+            String message = String.format("Duplicate shape definition for `%s` found at `%s` and `%s`",
+                                           id, previous.getSourceLocation(), source.getSourceLocation());
+            throw new SourceException(message, source);
+        }
     }
 
     /**