diff --git a/internal/api/rest.go b/internal/api/rest.go
index e689f56..fd62d75 100644
--- a/internal/api/rest.go
+++ b/internal/api/rest.go
@@ -57,6 +57,7 @@ type RestApi struct {
 	Authentication    *auth.Authentication
 	MachineStateDir   string
 	OngoingArchivings sync.WaitGroup
+	RepositoryMutex   sync.Mutex
 }
 
 func (api *RestApi) MountRoutes(r *mux.Router) {
@@ -362,6 +363,11 @@ func (api *RestApi) startJob(rw http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// aquire lock to avoid race condition between API calls
+	var unlockOnce sync.Once
+	api.RepositoryMutex.Lock()
+	defer unlockOnce.Do(api.RepositoryMutex.Unlock)
+
 	// Check if combination of (job_id, cluster_id, start_time) already exists:
 	jobs, err := api.JobRepository.FindAll(&req.JobID, &req.Cluster, nil)
 	if err != nil && err != sql.ErrNoRows {
@@ -381,6 +387,8 @@ func (api *RestApi) startJob(rw http.ResponseWriter, r *http.Request) {
 		handleError(fmt.Errorf("insert into database failed: %w", err), http.StatusInternalServerError, rw)
 		return
 	}
+	// unlock here, adding Tags can be async
+	unlockOnce.Do(api.RepositoryMutex.Unlock)
 
 	for _, tag := range req.Tags {
 		if _, err := api.JobRepository.AddTagOrCreate(id, tag.Type, tag.Name); err != nil {