diff --git a/.changeset/fix-plugin-initialization-idempotency.md b/.changeset/fix-plugin-initialization-idempotency.md
index db3f8086..9d91cc37 100644
--- a/.changeset/fix-plugin-initialization-idempotency.md
+++ b/.changeset/fix-plugin-initialization-idempotency.md
@@ -4,4 +4,5 @@
 
 Prevent plugins from initializing twice through `initializeCoco()`, and make
 plugin lifecycle hooks idempotent across repeated or concurrent init and ready
-calls.
+calls. Registering the same plugin instance more than once now throws instead
+of creating an unbalanced dispose lifecycle.
diff --git a/packages/core/plugins/PluginHost.ts b/packages/core/plugins/PluginHost.ts
index e2d5afc5..5a396242 100644
--- a/packages/core/plugins/PluginHost.ts
+++ b/packages/core/plugins/PluginHost.ts
@@ -1,10 +1,11 @@
 import type { Plugin, ServiceKey, ServiceMap } from './types.ts';
-import { ExtensionRegistrationError } from './types.ts';
+import { DuplicatePluginRegistrationError, ExtensionRegistrationError } from './types.ts';
 
 export class PluginHost {
   private readonly plugins: Plugin[] = [];
   private readonly cleanups: Array<() => void | Promise<void>> = [];
   private readonly extensions: Record<string, unknown> = {};
+  private readonly registeredPlugins = new WeakSet<Plugin>();
   private readonly initializedPlugins = new WeakSet<Plugin>();
   private readonly readyPlugins = new WeakSet<Plugin>();
   private readonly initPromises = new WeakMap<Plugin, Promise<void>>();
@@ -14,6 +15,11 @@ export class PluginHost {
   private readyPhase = false;
 
   use(plugin: Plugin): void {
+    if (this.registeredPlugins.has(plugin)) {
+      throw new DuplicatePluginRegistrationError(plugin.name);
+    }
+
+    this.registeredPlugins.add(plugin);
     this.plugins.push(plugin);
     if (this.initialized && this.services) {
       const services = this.services;
diff --git a/packages/core/plugins/types.ts b/packages/core/plugins/types.ts
index 395a563c..ad2b5e0d 100644
--- a/packages/core/plugins/types.ts
+++ b/packages/core/plugins/types.ts
@@ -90,3 +90,13 @@ export class ExtensionRegistrationError extends Error {
     this.name = 'ExtensionRegistrationError';
   }
 }
+
+/**
+ * Error thrown when the same plugin instance is registered more than once.
+ */
+export class DuplicatePluginRegistrationError extends Error {
+  constructor(pluginName: string) {
+    super(`Plugin "${pluginName}" is already registered`);
+    this.name = 'DuplicatePluginRegistrationError';
+  }
+}
diff --git a/packages/core/test/unit/Manager.test.ts b/packages/core/test/unit/Manager.test.ts
index ec9f801e..1ce666d9 100644
--- a/packages/core/test/unit/Manager.test.ts
+++ b/packages/core/test/unit/Manager.test.ts
@@ -331,6 +331,27 @@ describe('initializeCoco', () => {
       await manager.disableProofStateWatcher();
       await manager.disableMintOperationProcessor();
     });
+
+    it('should reject duplicate plugin instance registration', async () => {
+      const manager = await initializeCoco({
+        ...baseConfig,
+        watchers: {
+          mintOperationWatcher: { disabled: true },
+          proofStateWatcher: { disabled: true },
+        },
+        processors: {
+          mintOperationProcessor: { disabled: true },
+        },
+      });
+      const plugin = {
+        name: 'duplicate-plugin',
+        required: [] as const,
+      };
+
+      manager.use(plugin);
+
+      expect(() => manager.use(plugin)).toThrow('Plugin "duplicate-plugin" is already registered');
+    });
   });
 
   describe('edge cases', () => {
diff --git a/packages/core/test/unit/PluginHost.test.ts b/packages/core/test/unit/PluginHost.test.ts
index 2d0b6687..fccb651f 100644
--- a/packages/core/test/unit/PluginHost.test.ts
+++ b/packages/core/test/unit/PluginHost.test.ts
@@ -1,5 +1,6 @@
 import { describe, it, beforeEach, afterEach, expect } from 'bun:test';
 import { PluginHost } from '../../plugins/PluginHost.ts';
+import { DuplicatePluginRegistrationError } from '../../plugins/types.ts';
 import type { Plugin } from '../../plugins/types.ts';
 
 describe('PluginHost', () => {
@@ -93,6 +94,34 @@ describe('PluginHost', () => {
     expect(calls).toEqual({ init: 1, ready: 1 });
   });
 
+  it('rejects duplicate plugin instance registration', async () => {
+    const calls = { init: 0, ready: 0, dispose: 0 };
+    const plugin: Plugin<['logger']> = {
+      name: 'duplicate',
+      required: ['logger'],
+      onInit: () => {
+        calls.init += 1;
+      },
+      onReady: () => {
+        calls.ready += 1;
+      },
+      onDispose: () => {
+        calls.dispose += 1;
+      },
+    };
+
+    host.use(plugin);
+
+    expect(() => host.use(plugin)).toThrow(DuplicatePluginRegistrationError);
+    expect(() => host.use(plugin)).toThrow('Plugin "duplicate" is already registered');
+
+    await host.init(services);
+    await host.ready();
+    await host.dispose();
+
+    expect(calls).toEqual({ init: 1, ready: 1, dispose: 1 });
+  });
+
   it('coalesces concurrent init and ready calls', async () => {
     const calls = { init: 0, ready: 0 };
     const plugin: Plugin<['logger']> = {