diff --git a/schema/out/ts/ifcx.d.ts b/schema/out/ts/ifcx.d.ts
index 745fa2b..7aa9bda 100644
--- a/schema/out/ts/ifcx.d.ts
+++ b/schema/out/ts/ifcx.d.ts
@@ -24,6 +24,8 @@ export interface components {
                 [key: string]: components["schemas"]["IfcxSchema"];
             };
             data: components["schemas"]["IfcxNode"][];
+            // hack hack hack
+            src?: string;
         };
         IfcxHeader: {
             id: string;
diff --git a/src/ifcx-core/layers/layer-stack.ts b/src/ifcx-core/layers/layer-stack.ts
index 7a2b78c..f1afb44 100644
--- a/src/ifcx-core/layers/layer-stack.ts
+++ b/src/ifcx-core/layers/layer-stack.ts
@@ -88,13 +88,15 @@ export class IfcxLayerStackBuilder
     private async SatisfyDependencies(activeLayer: IfcxFile, placed: Map<string, boolean>, orderedLayers: IfcxFile[])
     {
         let pending: IfcxFile[] = [];
-        for (const impt of activeLayer.imports) {
+        for (const impt of activeLayer.imports || []) {
             if (!placed.has(impt.uri))
             {
                 let layer = await this.provider.GetLayerByURI(impt.uri);
                 if (layer instanceof Error)
                 {
                     return layer;
+                } else {
+                    layer.src = impt.uri;
                 }
                 pending.push(layer);
                 placed.set(impt.uri, true);
diff --git a/src/ifcx-core/workflows.ts b/src/ifcx-core/workflows.ts
index 23d56ad..185a90e 100644
--- a/src/ifcx-core/workflows.ts
+++ b/src/ifcx-core/workflows.ts
@@ -1,7 +1,7 @@
 import { FlattenCompositionInput, CreateArtificialRoot, ExpandFirstRootInInput } from "./composition/compose";
 import { CompositionInputNode } from "./composition/node";
 import { IfcxFile, IfcxNode, ImportNode } from "./schema/schema-helper";
-import { Validate } from "./schema/schema-validation";
+import { SchemaValidationError, Validate } from "./schema/schema-validation";
 import { MMSet } from "./util/mm";
 
 function ToInputNodes(data: IfcxNode[])
@@ -151,7 +151,7 @@ export function Diff(file1: IfcxFile, file2: IfcxFile)
     return result;
 }
 
-export function Federate(files: IfcxFile[])
+export function Federate(files: IfcxFile[], acceptDuplicateSchemas: boolean = false)
 {
     if (files.length === 0)
     {
@@ -165,6 +165,26 @@ export function Federate(files: IfcxFile[])
         data: []
     };
 
+    if (!acceptDuplicateSchemas) {
+        files.forEach((file) => {
+            // @todo not very efficient in case of deep import trees
+            const localSchemas = new Set(Object.keys(file.schemas));
+            const importedSchemas : Set<String> = new Set();
+            const recurse = (uri : string) => {
+                files.filter(f => f.src == uri).forEach(f => {
+                    Object.keys(f.schemas).forEach(s => importedSchemas.add(s));
+                    f.imports.map(i => i.uri).forEach(recurse);
+                });
+            };
+            file.imports.map(i => i.uri).forEach(recurse);
+            for (let s of localSchemas) {
+                if (importedSchemas.has(s)) {
+                    throw new SchemaValidationError(`Imported schema '${s}' also defined in base layer`);
+                }
+            }
+        })
+    }
+
     files.forEach((file) => {
         Object.keys(file.schemas).forEach((schemaID) => result.schemas[schemaID] = file.schemas[schemaID]);
     })