diff --git a/src/utils/Processing.ts b/src/utils/Processing.ts
index c3d7a7be..6dbebcb3 100644
--- a/src/utils/Processing.ts
+++ b/src/utils/Processing.ts
@@ -122,7 +122,9 @@ export async function loadExternalDependencies(
       (dep: fhirtypes.ImplementationGuideDependsOn) => `${dep.packageId}@${dep.version}`
     ) ?? [];
   const fhirPackageId = determineCorePackageId(config.config.fhirVersion[0]);
-  allDependencies.push(`${fhirPackageId}@${config.config.fhirVersion[0]}`);
+  if (!allDependencies.includes(`${fhirPackageId}@${config.config.fhirVersion[0]}`)) {
+    allDependencies.push(`${fhirPackageId}@${config.config.fhirVersion[0]}`);
+  }
   await utils.loadAutomaticDependencies(
     config.config.fhirVersion[0],
     config.config.dependencies ?? [],
diff --git a/test/api/FhirToFsh.test.ts b/test/api/FhirToFsh.test.ts
index 51140e4b..f1a75b49 100644
--- a/test/api/FhirToFsh.test.ts
+++ b/test/api/FhirToFsh.test.ts
@@ -2,7 +2,7 @@ import fs from 'fs-extra';
 import path from 'path';
 import * as processing from '../../src/utils/Processing';
 import { fshtypes } from 'fsh-sushi';
-import { logger } from '../../src/utils';
+import { FHIRDefinitions, logger } from '../../src/utils';
 import { fhirToFsh, ResourceMap } from '../../src/api';
 import { loggerSpy } from '../helpers/loggerSpy';
 import { EOL } from 'os';
@@ -111,6 +111,30 @@ describe('fhirToFsh', () => {
     expect(results.configuration).toEqual(defaultConfig);
   });
 
+  it('should try to load external dependencies', async () => {
+    await fhirToFsh([], {
+      dependencies: ['hl7.fhir.us.core#3.1.0', 'hl7.fhir.us.mcode@1.0.0']
+    });
+    expect(loadSpy).toHaveBeenCalledTimes(1);
+    expect(loadSpy).toHaveBeenCalledWith(
+      expect.any(FHIRDefinitions),
+      expect.objectContaining({
+        config: expect.objectContaining({
+          dependencies: [
+            {
+              packageId: 'hl7.fhir.us.core',
+              version: '3.1.0'
+            },
+            {
+              packageId: 'hl7.fhir.us.mcode',
+              version: '1.0.0'
+            }
+          ]
+        })
+      })
+    );
+  });
+
   it('should parse a string input into JSON', async () => {
     const results = await fhirToFsh([
       JSON.stringify({
diff --git a/test/utils/Processing.test.ts b/test/utils/Processing.test.ts
index 6a74462c..5d11bcc2 100644
--- a/test/utils/Processing.test.ts
+++ b/test/utils/Processing.test.ts
@@ -580,6 +580,29 @@ describe('Processing', () => {
       expect(loadedPackages).toContain('hl7.terminology.r4#1.0.0');
       expect(loggerSpy.getAllMessages('error')).toHaveLength(0);
     });
+
+    it('should not add the identified FHIR core package to the list of dependencies when it is already there', async () => {
+      const config = new ExportableConfiguration({
+        FSHOnly: true,
+        canonical: 'http://example.org',
+        fhirVersion: ['4.0.1'],
+        id: 'example',
+        name: 'Example',
+        applyExtensionMetadataToRoot: false,
+        dependencies: [
+          { packageId: 'hl7.fhir.us.core', version: '3.1.0' },
+          { packageId: 'hl7.fhir.r4.core', version: '4.0.1' }
+        ]
+      });
+      const defs = new FHIRDefinitions();
+      await loadExternalDependencies(defs, config);
+      // the core FHIR package is only present once in the list
+      expect(loadedPackages).toHaveLength(3);
+      expect(loadedPackages).toContain('hl7.fhir.r4.core#4.0.1');
+      expect(loadedPackages).toContain('hl7.fhir.us.core#3.1.0');
+      expect(loadedPackages).toContain('hl7.terminology.r4#1.0.0');
+      expect(loggerSpy.getAllMessages('error')).toHaveLength(0);
+    });
   });
 
   describe('loadConfiguredDependencies', () => {