diff --git a/src/core/cgroup.c b/src/core/cgroup.c
index 8db044befa..ad67ba0438 100644
--- a/src/core/cgroup.c
+++ b/src/core/cgroup.c
@@ -2319,7 +2319,7 @@ static int unit_watch_pids_in_path(Unit *u, const char *path) {
                 pid_t pid;
 
                 while ((r = cg_read_pid(f, &pid)) > 0) {
-                        r = unit_watch_pid(u, pid);
+                        r = unit_watch_pid(u, pid, false);
                         if (r < 0 && ret >= 0)
                                 ret = r;
                 }
diff --git a/src/core/dbus-scope.c b/src/core/dbus-scope.c
index bb807df2e9..8eb915e508 100644
--- a/src/core/dbus-scope.c
+++ b/src/core/dbus-scope.c
@@ -106,7 +106,7 @@ static int bus_scope_set_transient_property(
                                 return r;
 
                         if (!UNIT_WRITE_FLAGS_NOOP(flags)) {
-                                r = unit_watch_pid(UNIT(s), pid);
+                                r = unit_watch_pid(UNIT(s), pid, false);
                                 if (r < 0 && r != -EEXIST)
                                         return r;
                         }
diff --git a/src/core/manager.c b/src/core/manager.c
index 22d1fda2dc..33785d905f 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -2129,6 +2129,16 @@ void manager_clear_jobs(Manager *m) {
                 job_finish_and_invalidate(j, JOB_CANCELED, false, false);
 }
 
+void manager_unwatch_pid(Manager *m, pid_t pid) {
+        assert(m);
+
+        /* First let's drop the unit keyed as "pid". */
+        (void) hashmap_remove(m->watch_pids, PID_TO_PTR(pid));
+
+        /* Then, let's also drop the array keyed by -pid. */
+        free(hashmap_remove(m->watch_pids, PID_TO_PTR(-pid)));
+}
+
 static int manager_dispatch_run_queue(sd_event_source *source, void *userdata) {
         Manager *m = userdata;
         Job *j;
diff --git a/src/core/manager.h b/src/core/manager.h
index bce8020cfd..c43c587a79 100644
--- a/src/core/manager.h
+++ b/src/core/manager.h
@@ -437,6 +437,8 @@ int manager_get_dump_string(Manager *m, char **ret);
 
 void manager_clear_jobs(Manager *m);
 
+void manager_unwatch_pid(Manager *m, pid_t pid);
+
 unsigned manager_dispatch_load_queue(Manager *m);
 
 int manager_default_environment(Manager *m);
diff --git a/src/core/mount.c b/src/core/mount.c
index a991bf8794..4fb4b0f81e 100644
--- a/src/core/mount.c
+++ b/src/core/mount.c
@@ -704,7 +704,7 @@ static int mount_coldplug(Unit *u) {
             pid_is_unwaited(m->control_pid) &&
             MOUNT_STATE_WITH_PROCESS(new_state)) {
 
-                r = unit_watch_pid(UNIT(m), m->control_pid);
+                r = unit_watch_pid(UNIT(m), m->control_pid, false);
                 if (r < 0)
                         return r;
 
@@ -810,9 +810,8 @@ static int mount_spawn(Mount *m, ExecCommand *c, pid_t *_pid) {
         if (r < 0)
                 return r;
 
-        r = unit_watch_pid(UNIT(m), pid);
+        r = unit_watch_pid(UNIT(m), pid, true);
         if (r < 0)
-                /* FIXME: we need to do something here */
                 return r;
 
         *_pid = pid;
diff --git a/src/core/service.c b/src/core/service.c
index 4682e86f8e..883ca7bbfe 100644
--- a/src/core/service.c
+++ b/src/core/service.c
@@ -990,7 +990,7 @@ static int service_load_pid_file(Service *s, bool may_warn) {
         if (r < 0)
                 return r;
 
-        r = unit_watch_pid(UNIT(s), pid);
+        r = unit_watch_pid(UNIT(s), pid, false);
         if (r < 0) /* FIXME: we need to do something here */
                 return log_unit_warning_errno(UNIT(s), r, "Failed to watch PID "PID_FMT" for service: %m", pid);
 
@@ -1020,7 +1020,7 @@ static void service_search_main_pid(Service *s) {
         if (service_set_main_pid(s, pid) < 0)
                 return;
 
-        r = unit_watch_pid(UNIT(s), pid);
+        r = unit_watch_pid(UNIT(s), pid, false);
         if (r < 0)
                 /* FIXME: we need to do something here */
                 log_unit_warning_errno(UNIT(s), r, "Failed to watch PID "PID_FMT" from: %m", pid);
@@ -1154,7 +1154,7 @@ static int service_coldplug(Unit *u) {
                     SERVICE_RUNNING, SERVICE_RELOAD,
                     SERVICE_STOP, SERVICE_STOP_WATCHDOG, SERVICE_STOP_SIGTERM, SERVICE_STOP_SIGKILL, SERVICE_STOP_POST,
                     SERVICE_FINAL_SIGTERM, SERVICE_FINAL_SIGKILL))) {
-                r = unit_watch_pid(UNIT(s), s->main_pid);
+                r = unit_watch_pid(UNIT(s), s->main_pid, false);
                 if (r < 0)
                         return r;
         }
@@ -1166,7 +1166,7 @@ static int service_coldplug(Unit *u) {
                    SERVICE_RELOAD,
                    SERVICE_STOP, SERVICE_STOP_WATCHDOG, SERVICE_STOP_SIGTERM, SERVICE_STOP_SIGKILL, SERVICE_STOP_POST,
                    SERVICE_FINAL_SIGTERM, SERVICE_FINAL_SIGKILL)) {
-                r = unit_watch_pid(UNIT(s), s->control_pid);
+                r = unit_watch_pid(UNIT(s), s->control_pid, false);
                 if (r < 0)
                         return r;
         }
@@ -1566,8 +1566,8 @@ static int service_spawn(
         s->exec_fd_event_source = TAKE_PTR(exec_fd_source);
         s->exec_fd_hot = false;
 
-        r = unit_watch_pid(UNIT(s), pid);
-        if (r < 0) /* FIXME: we need to do something here */
+        r = unit_watch_pid(UNIT(s), pid, true);
+        if (r < 0)
                 return r;
 
         *_pid = pid;
@@ -3705,7 +3705,7 @@ static void service_notify_message(
                         if (r > 0) {
                                 service_set_main_pid(s, new_main_pid);
 
-                                r = unit_watch_pid(UNIT(s), new_main_pid);
+                                r = unit_watch_pid(UNIT(s), new_main_pid, false);
                                 if (r < 0)
                                         log_unit_warning_errno(UNIT(s), r, "Failed to watch new main PID "PID_FMT" for service: %m", new_main_pid);
 
@@ -3923,7 +3923,7 @@ static void service_bus_name_owner_change(
                         log_unit_debug(u, "D-Bus name %s is now owned by process " PID_FMT, name, pid);
 
                         service_set_main_pid(s, pid);
-                        unit_watch_pid(UNIT(s), pid);
+                        unit_watch_pid(UNIT(s), pid, false);
                 }
         }
 }
diff --git a/src/core/socket.c b/src/core/socket.c
index fd550290be..1396842b26 100644
--- a/src/core/socket.c
+++ b/src/core/socket.c
@@ -1852,7 +1852,7 @@ static int socket_coldplug(Unit *u) {
                    SOCKET_FINAL_SIGTERM,
                    SOCKET_FINAL_SIGKILL)) {
 
-                r = unit_watch_pid(UNIT(s), s->control_pid);
+                r = unit_watch_pid(UNIT(s), s->control_pid, false);
                 if (r < 0)
                         return r;
 
@@ -1938,9 +1938,8 @@ static int socket_spawn(Socket *s, ExecCommand *c, pid_t *_pid) {
         if (r < 0)
                 return r;
 
-        r = unit_watch_pid(UNIT(s), pid);
+        r = unit_watch_pid(UNIT(s), pid, true);
         if (r < 0)
-                /* FIXME: we need to do something here */
                 return r;
 
         *_pid = pid;
@@ -2009,7 +2008,7 @@ static int socket_chown(Socket *s, pid_t *_pid) {
                 _exit(EXIT_SUCCESS);
         }
 
-        r = unit_watch_pid(UNIT(s), pid);
+        r = unit_watch_pid(UNIT(s), pid, true);
         if (r < 0)
                 goto fail;
 
diff --git a/src/core/swap.c b/src/core/swap.c
index b836fdd304..28acef2373 100644
--- a/src/core/swap.c
+++ b/src/core/swap.c
@@ -550,7 +550,7 @@ static int swap_coldplug(Unit *u) {
             pid_is_unwaited(s->control_pid) &&
             SWAP_STATE_WITH_PROCESS(new_state)) {
 
-                r = unit_watch_pid(UNIT(s), s->control_pid);
+                r = unit_watch_pid(UNIT(s), s->control_pid, false);
                 if (r < 0)
                         return r;
 
@@ -657,9 +657,8 @@ static int swap_spawn(Swap *s, ExecCommand *c, pid_t *_pid) {
         if (r < 0)
                 goto fail;
 
-        r = unit_watch_pid(UNIT(s), pid);
+        r = unit_watch_pid(UNIT(s), pid, true);
         if (r < 0)
-                /* FIXME: we need to do something here */
                 goto fail;
 
         *_pid = pid;
diff --git a/src/core/unit.c b/src/core/unit.c
index 445cf6695c..5620fd9017 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -2525,7 +2525,7 @@ void unit_notify(Unit *u, UnitActiveState os, UnitActiveState ns, UnitNotifyFlag
         unit_add_to_gc_queue(u);
 }
 
-int unit_watch_pid(Unit *u, pid_t pid) {
+int unit_watch_pid(Unit *u, pid_t pid, bool exclusive) {
         int r;
 
         assert(u);
@@ -2533,6 +2533,12 @@ int unit_watch_pid(Unit *u, pid_t pid) {
 
         /* Watch a specific PID */
 
+        /* Caller might be sure that this PID belongs to this unit only. Let's take this
+         * opportunity to remove any stalled references to this PID as they can be created
+         * easily (when watching a process which is not our direct child). */
+        if (exclusive)
+                manager_unwatch_pid(u->manager, pid);
+
         r = set_ensure_allocated(&u->pids, NULL);
         if (r < 0)
                 return r;
diff --git a/src/core/unit.h b/src/core/unit.h
index 35b936771c..28878b5f4f 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -678,7 +678,7 @@ typedef enum UnitNotifyFlags {
 
 void unit_notify(Unit *u, UnitActiveState os, UnitActiveState ns, UnitNotifyFlags flags);
 
-int unit_watch_pid(Unit *u, pid_t pid);
+int unit_watch_pid(Unit *u, pid_t pid, bool exclusive);
 void unit_unwatch_pid(Unit *u, pid_t pid);
 void unit_unwatch_all_pids(Unit *u);
 
diff --git a/src/test/test-watch-pid.c b/src/test/test-watch-pid.c
index 2c6ca0a1a2..04d7e1fbd8 100644
--- a/src/test/test-watch-pid.c
+++ b/src/test/test-watch-pid.c
@@ -42,25 +42,25 @@ int main(int argc, char *argv[]) {
         assert_se(hashmap_isempty(m->watch_pids));
         assert_se(manager_get_unit_by_pid(m, 4711) == NULL);
 
-        assert_se(unit_watch_pid(a, 4711) >= 0);
+        assert_se(unit_watch_pid(a, 4711, false) >= 0);
         assert_se(manager_get_unit_by_pid(m, 4711) == a);
 
-        assert_se(unit_watch_pid(a, 4711) >= 0);
+        assert_se(unit_watch_pid(a, 4711, false) >= 0);
         assert_se(manager_get_unit_by_pid(m, 4711) == a);
 
-        assert_se(unit_watch_pid(b, 4711) >= 0);
+        assert_se(unit_watch_pid(b, 4711, false) >= 0);
         u = manager_get_unit_by_pid(m, 4711);
         assert_se(u == a || u == b);
 
-        assert_se(unit_watch_pid(b, 4711) >= 0);
+        assert_se(unit_watch_pid(b, 4711, false) >= 0);
         u = manager_get_unit_by_pid(m, 4711);
         assert_se(u == a || u == b);
 
-        assert_se(unit_watch_pid(c, 4711) >= 0);
+        assert_se(unit_watch_pid(c, 4711, false) >= 0);
         u = manager_get_unit_by_pid(m, 4711);
         assert_se(u == a || u == b || u == c);
 
-        assert_se(unit_watch_pid(c, 4711) >= 0);
+        assert_se(unit_watch_pid(c, 4711, false) >= 0);
         u = manager_get_unit_by_pid(m, 4711);
         assert_se(u == a || u == b || u == c);