diff --git a/internal/metadata/project.go b/internal/metadata/project.go
index cecca2578..44ddba168 100644
--- a/internal/metadata/project.go
+++ b/internal/metadata/project.go
@@ -3,10 +3,14 @@ package metadata
 import (
 	"context"
 	"database/sql"
+	"errors"
 	"fmt"
 	"sync"
 )
 
+var ErrDuplicatedDataset = errors.New("dataset is already created")
+var ErrDuplicatedJob = errors.New("job is already created")
+
 type Project struct {
 	ID         string
 	datasets   []*Dataset
@@ -69,7 +73,7 @@ func (p *Project) AddDataset(ctx context.Context, tx *sql.Tx, dataset *Dataset)
 	p.mu.Lock()
 	defer p.mu.Unlock()
 	if _, exists := p.datasetMap[dataset.ID]; exists {
-		return fmt.Errorf("dataset %s is already created", dataset.ID)
+		return fmt.Errorf("dataset %s: %w", dataset.ID, ErrDuplicatedDataset)
 	}
 	if err := dataset.Insert(ctx, tx); err != nil {
 		return err
@@ -111,7 +115,7 @@ func (p *Project) AddJob(ctx context.Context, tx *sql.Tx, job *Job) error {
 	p.mu.Lock()
 	defer p.mu.Unlock()
 	if _, exists := p.jobMap[job.ID]; exists {
-		return fmt.Errorf("job %s is already created", job.ID)
+		return ErrDuplicatedJob
 	}
 	if err := job.Insert(ctx, tx); err != nil {
 		return err
diff --git a/server/handler.go b/server/handler.go
index 692b66470..cd7c7a2ac 100644
--- a/server/handler.go
+++ b/server/handler.go
@@ -680,6 +680,10 @@ func (h *datasetsInsertHandler) ServeHTTP(w http.ResponseWriter, r *http.Request
 		dataset: &dataset,
 	})
 	if err != nil {
+		if errors.Is(err, metadata.ErrDuplicatedDataset) {
+			errDuplicate(err.Error())
+			return
+		}
 		errorResponse(ctx, w, errInternalError(err.Error()))
 		return
 	}
@@ -723,6 +727,9 @@ func (h *datasetsInsertHandler) Handle(ctx context.Context, r *datasetsInsertReq
 			nil,
 		),
 	); err != nil {
+		if errors.Is(err, metadata.ErrDuplicatedDataset) {
+			return nil, errDuplicate(err.Error())
+		}
 		return nil, err
 	}
 	if err := tx.Commit(); err != nil {