diff --git a/src/journal/journalctl-varlink-server.c b/src/journal/journalctl-varlink-server.c
index 8c662211a9..e1e2705a69 100644
--- a/src/journal/journalctl-varlink-server.c
+++ b/src/journal/journalctl-varlink-server.c
@@ -142,6 +142,10 @@ static int follow_state_setup(sd_varlink *link, sd_journal *_j /* donated! */) {
         assert(link);
         assert(j);
 
+        /* The varlink connection already requested the follow mode. Refusing. */
+        if (sd_varlink_get_userdata(link))
+                return -EBUSY;
+
         sd_varlink_server *server = ASSERT_PTR(sd_varlink_get_server(link));
         sd_event *event = ASSERT_PTR(sd_varlink_server_get_event(server));
 
diff --git a/src/journal/journald-manager.c b/src/journal/journald-manager.c
index be3532f1ed..a0fc0d8c9b 100644
--- a/src/journal/journald-manager.c
+++ b/src/journal/journald-manager.c
@@ -1873,7 +1873,7 @@ static int manager_schedule_sync(Manager *m, int priority) {
         }
 
         if (!m->event || sd_event_get_state(m->event) == SD_EVENT_FINISHED) {
-                /* Shutting down the server? Let's sync immediately. */
+                /* Shutting down the manager? Let's sync immediately. */
                 manager_sync(m, /* wait= */ false);
                 return 0;
         }
@@ -2133,7 +2133,7 @@ int manager_unlink_seqnum_file(Manager *m, const char *fname) {
 static bool manager_is_idle(Manager *m) {
         assert(m);
 
-        /* The server for the main namespace is never idle */
+        /* The manager for the main namespace is never idle */
         if (!m->namespace)
                 return false;
 
diff --git a/src/journal/journald-stream.c b/src/journal/journald-stream.c
index ee903755cf..8a93b2ef79 100644
--- a/src/journal/journald-stream.c
+++ b/src/journal/journald-stream.c
@@ -687,7 +687,7 @@ static int stdout_stream_new(sd_event_source *es, int listen_fd, uint32_t revent
 
         if (revents != EPOLLIN)
                 return log_error_errno(SYNTHETIC_ERRNO(EIO),
-                                       "Got invalid event from epoll for stdout server fd: %" PRIx32,
+                                       "Got invalid event from epoll for stdout fd: %" PRIx32,
                                        revents);
 
         fd = accept4(m->stdout_fd, NULL, NULL, SOCK_NONBLOCK|SOCK_CLOEXEC);
@@ -926,11 +926,11 @@ int manager_open_stdout_socket(Manager *m, const char *stdout_socket) {
 
         r = sd_event_add_io(m->event, &m->stdout_event_source, m->stdout_fd, EPOLLIN, stdout_stream_new, m);
         if (r < 0)
-                return log_error_errno(r, "Failed to add stdout server fd to event source: %m");
+                return log_error_errno(r, "Failed to add stdout fd to event source: %m");
 
         r = sd_event_source_set_priority(m->stdout_event_source, SD_EVENT_PRIORITY_NORMAL+5);
         if (r < 0)
-                return log_error_errno(r, "Failed to adjust priority of stdout server event source: %m");
+                return log_error_errno(r, "Failed to adjust priority of stdout event source: %m");
 
         return 0;
 }
diff --git a/src/journal/journald-sync.c b/src/journal/journald-sync.c
index 4722cfa9a3..45bb84fd83 100644
--- a/src/journal/journald-sync.c
+++ b/src/journal/journald-sync.c
@@ -17,7 +17,7 @@
 #include "socket-netlink.h"
 #include "time-util.h"
 
-StreamSyncReq *stream_sync_req_free(StreamSyncReq *ssr) {
+StreamSyncReq* stream_sync_req_free(StreamSyncReq *ssr) {
         if (!ssr)
                 return NULL;
 
diff --git a/src/journal/journald-sync.h b/src/journal/journald-sync.h
index e1172611e9..9ce77f9d1c 100644
--- a/src/journal/journald-sync.h
+++ b/src/journal/journald-sync.h
@@ -37,7 +37,7 @@ typedef struct SyncReq {
         unsigned boottime_prioq_idx;
 } SyncReq;
 
-StreamSyncReq *stream_sync_req_free(StreamSyncReq *ssr);
+StreamSyncReq* stream_sync_req_free(StreamSyncReq *ssr);
 DEFINE_TRIVIAL_CLEANUP_FUNC(StreamSyncReq*, stream_sync_req_free);
 void stream_sync_req_advance_revalidate(StreamSyncReq *ssr, size_t p);
 
diff --git a/src/journal/journald-varlink.c b/src/journal/journald-varlink.c
index 674f2524eb..cbadbca777 100644
--- a/src/journal/journald-varlink.c
+++ b/src/journal/journald-varlink.c
@@ -40,11 +40,17 @@ static int vl_method_synchronize(sd_varlink *link, sd_json_variant *parameters,
                 {}
         };
 
-        Manager *m = ASSERT_PTR(userdata);
         int r;
 
         assert(link);
 
+        /* Here, userdata may not be a pointer to the Manager object. Obtain the manager from the server. */
+        Manager *m = ASSERT_PTR(sd_varlink_server_get_userdata(sd_varlink_get_server(link)));
+
+        /* The varlink connection already requested to sync journals. Refusing. */
+        if (sd_varlink_get_userdata(link) != m)
+                return -EBUSY;
+
         r = sd_varlink_dispatch(link, parameters, dispatch_table, &offline);
         if (r != 0)
                 return r;
@@ -168,10 +174,11 @@ static void vl_disconnect(sd_varlink_server *server, sd_varlink *link, void *use
 
         void *u = sd_varlink_get_userdata(link);
         if (u != m) {
-                /* If this is a Varlink connection that does not have the Server object as userdata, then it has a SyncReq object instead. Let's finish it. */
+                /* If this is a Varlink connection that does not have the Manager object as userdata, then
+                 * it has a SyncReq object instead. Let's finish it. */
 
                 SyncReq *req = u;
-                sd_varlink_set_userdata(link, m); /* reinstall server object */
+                sd_varlink_set_userdata(link, m); /* reinstall the manager object */
                 sync_req_free(req);
         }
 
diff --git a/src/mute-console/mute-console.c b/src/mute-console/mute-console.c
index 7ea1fcf17b..0a5ef85ec2 100644
--- a/src/mute-console/mute-console.c
+++ b/src/mute-console/mute-console.c
@@ -236,6 +236,10 @@ static int vl_method_mute(
         assert(link);
         assert(FLAGS_SET(flags, SD_VARLINK_METHOD_MORE));
 
+        /* Refuse multiple requests per connection. */
+        if (sd_varlink_get_userdata(link))
+                return -EBUSY;
+
         _cleanup_free_ Context *nc = new(Context, 1);
         if (!nc)
                 return -ENOMEM;
diff --git a/src/repart/repart-list-candidate-devices.c b/src/repart/repart-list-candidate-devices.c
index f4f5a8aa2c..550064cb64 100644
--- a/src/repart/repart-list-candidate-devices.c
+++ b/src/repart/repart-list-candidate-devices.c
@@ -169,6 +169,10 @@ int vl_method_list_candidate_devices(
         _cleanup_(list_candidate_devices_context_freep) ListCandidateDevicesContext *c = NULL;
 
         if (p.subscribe) {
+                /* Refuse multiple requests per connection. */
+                if (sd_varlink_get_userdata(link))
+                        return -EBUSY;
+
                 /* Start the monitor *before* the initial enumeration so we don't lose events that fire
                  * during the enumeration window. Duplicate add events for the same device are
                  * legitimate and documented; clients upsert by identifier. */
diff --git a/src/resolve/resolved-dns-browse-services.c b/src/resolve/resolved-dns-browse-services.c
index 445ab974f1..c25180d99b 100644
--- a/src/resolve/resolved-dns-browse-services.c
+++ b/src/resolve/resolved-dns-browse-services.c
@@ -726,27 +726,30 @@ static int mdns_next_query_schedule(sd_event_source *s, uint64_t usec, void *use
         assert(userdata);
         assert_se(sb = dns_service_browser_ref(userdata));
 
-        /* Enable the answer from the cache for the very first query */
-        if (sb->delay == 0)
-                SET_FLAG(sb->flags, SD_RESOLVED_NO_CACHE, false);
+        /* If the varlink connection has a userdata, then that means the previous query has not been finished. */
+        if (!sd_varlink_get_userdata(sb->link)) {
 
-        /* Set the flag indicating that the query is continuous.
-         * RFC 6762 Section 5.2 outlines timing requirements for continuous queries.
-         */
-        sb->flags |= SD_RESOLVED_QUERY_CONTINUOUS;
+                /* Enable the answer from the cache for the very first query */
+                if (sb->delay == 0)
+                        SET_FLAG(sb->flags, SD_RESOLVED_NO_CACHE, false);
 
-        r = dns_query_new(sb->manager, &q, sb->question_utf8, sb->question_idna, NULL, sb->ifindex, sb->flags);
-        if (r < 0)
-                return log_error_errno(r, "Failed to create new DNS query: %m");
+                /* Set the flag indicating that the query is continuous.
+                 * RFC 6762 Section 5.2 outlines timing requirements for continuous queries. */
+                sb->flags |= SD_RESOLVED_QUERY_CONTINUOUS;
 
-        q->complete = mdns_browse_service_query_complete;
-        q->service_browser_request = dns_service_browser_ref(sb);
-        q->varlink_request = sd_varlink_ref(sb->link);
-        sd_varlink_set_userdata(sb->link, q);
+                r = dns_query_new(sb->manager, &q, sb->question_utf8, sb->question_idna, NULL, sb->ifindex, sb->flags);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to create new DNS query: %m");
 
-        r = dns_query_go(q);
-        if (r < 0)
-                return log_error_errno(r, "Failed to send DNS query: %m");
+                q->complete = mdns_browse_service_query_complete;
+                q->service_browser_request = dns_service_browser_ref(sb);
+                q->varlink_request = sd_varlink_ref(sb->link);
+                sd_varlink_set_userdata(sb->link, q);
+
+                r = dns_query_go(q);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to send DNS query: %m");
+        }
 
         /* Calculate the next query delay */
         sb->delay = mdns_calculate_next_query_delay(sb->delay);
@@ -811,6 +814,10 @@ int dns_subscribe_browse_service(
         assert(m);
         assert(link);
 
+        /* Refuse multiple requests. */
+        if (hashmap_contains(m->dns_service_browsers, link))
+                return -EBUSY;
+
         if (ifindex < 0)
                 return sd_varlink_error_invalid_parameter_name(link, "ifindex");
 
diff --git a/src/resolve/resolved-varlink.c b/src/resolve/resolved-varlink.c
index 2617b62775..1ef4e2591d 100644
--- a/src/resolve/resolved-varlink.c
+++ b/src/resolve/resolved-varlink.c
@@ -363,6 +363,9 @@ static int vl_method_resolve_hostname(sd_varlink *link, sd_json_variant *paramet
 
         assert(link);
 
+        if (sd_varlink_get_userdata(link))
+                return -EBUSY;
+
         m = sd_varlink_server_get_userdata(sd_varlink_get_server(link));
         assert(m);
 
@@ -490,6 +493,9 @@ static int vl_method_resolve_address(sd_varlink *link, sd_json_variant *paramete
 
         assert(link);
 
+        if (sd_varlink_get_userdata(link))
+                return -EBUSY;
+
         m = sd_varlink_server_get_userdata(sd_varlink_get_server(link));
         assert(m);
 
@@ -973,6 +979,9 @@ static int vl_method_resolve_service(sd_varlink* link, sd_json_variant* paramete
 
         assert(link);
 
+        if (sd_varlink_get_userdata(link))
+                return -EBUSY;
+
         m = sd_varlink_server_get_userdata(sd_varlink_get_server(link));
         assert(m);
 
@@ -1124,6 +1133,9 @@ static int vl_method_resolve_record(sd_varlink *link, sd_json_variant *parameter
 
         assert(link);
 
+        if (sd_varlink_get_userdata(link))
+                return -EBUSY;
+
         m = ASSERT_PTR(sd_varlink_server_get_userdata(sd_varlink_get_server(link)));
 
         if (FLAGS_SET(flags, SD_VARLINK_METHOD_ONEWAY))
diff --git a/src/sysinstall/sysinstall.c b/src/sysinstall/sysinstall.c
index bfeb0f475c..d981be03d0 100644
--- a/src/sysinstall/sysinstall.c
+++ b/src/sysinstall/sysinstall.c
@@ -1822,6 +1822,10 @@ static int vl_method_list_candidate_devices(
 
         assert(link);
 
+        /* Refuse multiple requests */
+        if (sd_varlink_get_userdata(link))
+                return -EBUSY;
+
         sd_varlink_server *varlink_server = sd_varlink_get_server(link);
         sd_event *event = sd_varlink_server_get_event(varlink_server);
         Hashmap **polkit_registry = ASSERT_PTR(sd_varlink_server_get_userdata(varlink_server));