diff --git a/packages/loader/container-loader/src/container.ts b/packages/loader/container-loader/src/container.ts
index 02645b07a478..bc4bc2f5e816 100644
--- a/packages/loader/container-loader/src/container.ts
+++ b/packages/loader/container-loader/src/container.ts
@@ -11,6 +11,7 @@ import {
     ITelemetryBaseLogger,
     ITelemetryLogger,
 } from "@fluidframework/common-definitions";
+import { performance } from "@fluidframework/common-utils";
 import { IFluidObject, IRequest, IResponse, IFluidRouter } from "@fluidframework/core-interfaces";
 import {
     IAudience,
@@ -29,14 +30,7 @@ import {
     IThrottlingWarning,
     AttachState,
 } from "@fluidframework/container-definitions";
-import { performance } from "@fluidframework/common-utils";
-import {
-    ChildLogger,
-    EventEmitterWithErrorHandling,
-    PerformanceEvent,
-    raiseConnectedEvent,
-    TelemetryLogger,
-} from "@fluidframework/telemetry-utils";
+import { CreateContainerError, GenericError } from "@fluidframework/container-utils";
 import {
     IDocumentService,
     IDocumentStorageService,
@@ -56,7 +50,6 @@ import {
     combineAppAndProtocolSummary,
     readAndParseFromBlobs,
 } from "@fluidframework/driver-utils";
-import { CreateContainerError } from "@fluidframework/container-utils";
 import {
     isSystemMessage,
     ProtocolOpHandler,
@@ -85,6 +78,13 @@ import {
     TreeEntry,
     ISummaryTree,
 } from "@fluidframework/protocol-definitions";
+import {
+    ChildLogger,
+    EventEmitterWithErrorHandling,
+    PerformanceEvent,
+    raiseConnectedEvent,
+    TelemetryLogger,
+} from "@fluidframework/telemetry-utils";
 import { Audience } from "./audience";
 import { ContainerContext } from "./containerContext";
 import { debug } from "./debug";
@@ -98,6 +98,10 @@ import { parseUrl, convertProtocolAndAppSummaryToSnapshotTree } from "./utils";
 
 const PackageNotFactoryError = "Code package does not implement IRuntimeFactory";
 
+interface ILocalSequencedClient extends ISequencedClient {
+    shouldHaveLeft?: boolean;
+}
+
 export interface IContainerConfig {
     resolvedUrl?: IResolvedUrl;
     canReconnect?: boolean;
@@ -1391,6 +1395,14 @@ export class Container extends EventEmitterWithErrorHandling<IContainerEvents> i
         this._connectionState = value;
 
         if (value === ConnectionState.Connected) {
+            // Mark our old client should have left in the quorum if it's still there
+            if (this._clientId !== undefined) {
+                const client: ILocalSequencedClient | undefined =
+                    this._protocolHandler?.quorum.getMember(this._clientId);
+                if (client !== undefined) {
+                    client.shouldHaveLeft = true;
+                }
+            }
             this._clientId = this.pendingClientId;
             this._deltaManager.updateQuorumJoin();
         } else if (value === ConnectionState.Disconnected) {
@@ -1451,6 +1463,31 @@ export class Container extends EventEmitterWithErrorHandling<IContainerEvents> i
     }
 
     private processRemoteMessage(message: ISequencedDocumentMessage): IProcessMessageResult {
+        // Check and report if we're getting messages from a clientId that we previously
+        // flagged as shouldHaveLeft, or from a client that's not in the quorum but should be
+        if (message.clientId != null) {
+            let errorMsg: string | undefined;
+            const client: ILocalSequencedClient | undefined =
+                this._protocolHandler?.quorum.getMember(message.clientId);
+            if (client === undefined && message.type !== MessageType.ClientJoin) {
+                errorMsg = "messageClientIdMissingFromQuorum";
+            } else if (client?.shouldHaveLeft === true) {
+                errorMsg = "messageClientIdShouldHaveLeft";
+            }
+            if (errorMsg !== undefined) {
+                const error = new GenericError(
+                    errorMsg,
+                    {
+                        clientId: this._clientId,
+                        messageClientId: message.clientId,
+                        sequenceNumber: message.sequenceNumber,
+                        clientSequenceNumber: message.clientSequenceNumber,
+                    },
+                );
+                this.close(CreateContainerError(error));
+            }
+        }
+
         const local = this._clientId === message.clientId;
 
         // Forward non system messages to the loaded runtime for processing
diff --git a/packages/loader/container-loader/src/deltaManager.ts b/packages/loader/container-loader/src/deltaManager.ts
index 346b9e5dc14a..a7f812c8e20b 100644
--- a/packages/loader/container-loader/src/deltaManager.ts
+++ b/packages/loader/container-loader/src/deltaManager.ts
@@ -891,6 +891,8 @@ export class DeltaManager
      * @param connection - The newly established connection
      */
     private setupNewSuccessfulConnection(connection: DeltaConnection, requestedMode: ConnectionMode) {
+        // Old connection should have been cleaned up before establishing a new one
+        assert(this.connection === undefined, "old connection exists on new connection setup");
         this.connection = connection;
 
         // Does information in scopes & mode matches?
diff --git a/packages/loader/container-utils/src/error.ts b/packages/loader/container-utils/src/error.ts
index 887f8891354b..caab34bbee4f 100644
--- a/packages/loader/container-utils/src/error.ts
+++ b/packages/loader/container-utils/src/error.ts
@@ -17,7 +17,7 @@ function messageFromError(error: any) {
 /**
  * Generic error
  */
-class GenericError extends CustomErrorWithProps implements IGenericError {
+export class GenericError extends CustomErrorWithProps implements IGenericError {
     readonly errorType = ContainerErrorType.genericError;
 
     constructor(