diff --git a/ChangeLog.txt b/ChangeLog.txt
index 45c1a1dd..bf7e0fb2 100644
--- a/ChangeLog.txt
+++ b/ChangeLog.txt
@@ -7,6 +7,8 @@ Broker:
 - Fix retained messages not sent by bridges on outgoing topics at the first
   connection. Closes #701.
 - Minor documentation fixes. Closes #520.
+- Fix duplicate clients being added to by_id hash before the old client was
+  removed. Closes #645.
 
 Build:
 - Various fixes to ease building.
diff --git a/lib/mosquitto_internal.h b/lib/mosquitto_internal.h
index de82ec3d..c6b3d6e5 100644
--- a/lib/mosquitto_internal.h
+++ b/lib/mosquitto_internal.h
@@ -208,6 +208,8 @@ struct mosquitto {
 #endif
 	bool clean_session;
 #ifdef WITH_BROKER
+	char *old_id; /* for when a duplicate client connects, but we still want to
+					 know what the id was */
 	bool is_dropping;
 	bool is_bridge;
 	struct mosquitto__bridge *bridge;
diff --git a/src/context.c b/src/context.c
index 495693e9..1d3ae163 100644
--- a/src/context.c
+++ b/src/context.c
@@ -161,6 +161,10 @@ void context__cleanup(struct mosquitto_db *db, struct mosquitto *context, bool d
 		mosquitto__free(context->id);
 		context->id = NULL;
 	}
+	if(context->old_id){
+		mosquitto__free(context->old_id);
+		context->old_id = NULL;
+	}
 	packet__cleanup(&(context->in_packet));
 	if(context->current_out_packet){
 		packet__cleanup(context->current_out_packet);
diff --git a/src/loop.c b/src/loop.c
index 8df41760..97147209 100644
--- a/src/loop.c
+++ b/src/loop.c
@@ -638,12 +638,17 @@ void do_disconnect(struct mosquitto_db *db, struct mosquitto *context)
 			context->sock = INVALID_SOCKET;
 			context->pollfd_index = -1;
 		}
+		HASH_DELETE(hh_id, db->contexts_by_id, context);
+		context->old_id = context->id;
+		context->id = NULL;
 	}else
 #endif
 	{
 		if(db->config->connection_messages == true){
 			if(context->id){
 				id = context->id;
+			}else if(context->old_id){
+				id = context->old_id;
 			}else{
 				id = "<unknown>";
 			}