diff --git a/src/core/dbus-manager.c b/src/core/dbus-manager.c
index bdbaa9b58c..633873da27 100644
--- a/src/core/dbus-manager.c
+++ b/src/core/dbus-manager.c
@@ -2171,6 +2171,11 @@ static int install_error(
                                               "Unit %s is transient or generated.", changes[i].path);
                         goto found;
 
+                case -ETXTBSY:
+                        r = sd_bus_error_setf(error, BUS_ERROR_UNIT_BAD_PATH,
+                                              "File %s is under the systemd unit hierarchy already.", changes[i].path);
+                        goto found;
+
                 case -EUCLEAN:
                         r = sd_bus_error_setf(error, BUS_ERROR_BAD_UNIT_SETTING,
                                               "\"%s\" is not a valid unit name.",
diff --git a/src/libsystemd/sd-bus/bus-common-errors.h b/src/libsystemd/sd-bus/bus-common-errors.h
index c3c25d69c3..d4a1fb689e 100644
--- a/src/libsystemd/sd-bus/bus-common-errors.h
+++ b/src/libsystemd/sd-bus/bus-common-errors.h
@@ -20,6 +20,7 @@
 #define BUS_ERROR_UNIT_MASKED                  "org.freedesktop.systemd1.UnitMasked"
 #define BUS_ERROR_UNIT_GENERATED               "org.freedesktop.systemd1.UnitGenerated"
 #define BUS_ERROR_UNIT_LINKED                  "org.freedesktop.systemd1.UnitLinked"
+#define BUS_ERROR_UNIT_BAD_PATH                "org.freedesktop.systemd1.UnitBadPath"
 #define BUS_ERROR_JOB_TYPE_NOT_APPLICABLE      "org.freedesktop.systemd1.JobTypeNotApplicable"
 #define BUS_ERROR_NO_ISOLATION                 "org.freedesktop.systemd1.NoIsolation"
 #define BUS_ERROR_SHUTTING_DOWN                "org.freedesktop.systemd1.ShuttingDown"
diff --git a/src/shared/install.c b/src/shared/install.c
index cca8b0b0c8..6d91a02f11 100644
--- a/src/shared/install.c
+++ b/src/shared/install.c
@@ -104,13 +104,19 @@ static int in_search_path(const LookupPaths *lp, const char *path) {
         _cleanup_free_ char *parent = NULL;
         int r;
 
-        assert(path);
+        /* Check if 'path' is in lp->search_path. */
 
-        r = path_extract_directory(path, &parent);
+        r = path_extract_directory(ASSERT_PTR(path), &parent);
         if (r < 0)
                 return r;
 
-        return path_strv_contains(lp->search_path, parent);
+        return path_strv_contains(ASSERT_PTR(lp)->search_path, parent);
+}
+
+static int underneath_search_path(const LookupPaths *lp, const char *path) {
+        /* Check if 'path' is underneath lp->search_path. */
+
+        return !!path_startswith_strv(ASSERT_PTR(path), ASSERT_PTR(lp)->search_path);
 }
 
 static const char* skip_root(const char *root_dir, const char *path) {
@@ -390,6 +396,10 @@ void install_changes_dump(int r, const char *verb, const InstallChange *changes,
                         err = log_error_errno(changes[i].type, "Failed to %s unit, unit %s is transient or generated.",
                                               verb, changes[i].path);
                         break;
+                case -ETXTBSY:
+                        err = log_error_errno(changes[i].type, "Failed to %s unit, file %s is under the systemd unit hierarchy already.",
+                                              verb, changes[i].path);
+                        break;
                 case -EBADSLT:
                         err = log_error_errno(changes[i].type, "Failed to %s unit, invalid specifier in \"%s\".",
                                               verb, changes[i].path);
@@ -2425,8 +2435,15 @@ int unit_file_link(
                 if (r < 0)
                         return install_changes_add(changes, n_changes, r, *file, NULL);
                 if (r > 0)
+                        /* A silent noop if the file is already in the search path. */
                         continue;
 
+                r = underneath_search_path(&lp, *file);
+                if (r > 0)
+                        r = -ETXTBSY;
+                if (r < 0)
+                        return install_changes_add(changes, n_changes, r, *file, NULL);
+
                 if (!GREEDY_REALLOC0(todo, n_todo + 2))
                         return -ENOMEM;