diff --git a/pkg/index/compliance/compliance.go b/pkg/index/compliance/compliance.go
index 6c1fdf1b..16f46499 100644
--- a/pkg/index/compliance/compliance.go
+++ b/pkg/index/compliance/compliance.go
@@ -6,12 +6,16 @@ import (
 	"testing"
 	"time"
 
+	"github.com/gomods/athens/pkg/errors"
 	"github.com/gomods/athens/pkg/index"
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
 	"github.com/technosophos/moniker"
 )
 
+// RunTests runs compliance tests for the given Indexer implementation.
+// clearIndex is a function that must clear the entire storage so that
+// tests can assume a clean state.
 func RunTests(t *testing.T, indexer index.Indexer, clearIndex func() error) {
 	if err := clearIndex(); err != nil {
 		t.Fatal(err)
@@ -80,6 +84,23 @@ func RunTests(t *testing.T, indexer index.Indexer, clearIndex func() error) {
 			},
 			limit: 0,
 		},
+		{
+			name: "duplicate module version",
+			desc: "if we try to index a module that already exists, a KindAlreadyExists must be returned",
+			preTest: func(t *testing.T) ([]*index.Line, time.Time) {
+				m := &index.Line{Path: "gomods.io/tobeduplicated", Version: "v0.1.0"}
+				err := indexer.Index(context.Background(), m.Path, m.Version)
+				if err != nil {
+					t.Fatal(err)
+				}
+				err = indexer.Index(context.Background(), m.Path, m.Version)
+				if !errors.Is(err, errors.KindAlreadyExists) {
+					t.Fatalf("expected an error of kind AlreadyExists but got %s", errors.KindText(err))
+				}
+				return []*index.Line{m}, time.Time{}
+			},
+			limit: 2000,
+		},
 	}
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
diff --git a/pkg/index/mem/mem.go b/pkg/index/mem/mem.go
index 4ffb5ffc..253f0d43 100644
--- a/pkg/index/mem/mem.go
+++ b/pkg/index/mem/mem.go
@@ -2,6 +2,7 @@ package mem
 
 import (
 	"context"
+	"fmt"
 	"sync"
 	"time"
 
@@ -22,12 +23,17 @@ type indexer struct {
 func (i *indexer) Index(ctx context.Context, mod, ver string) error {
 	const op errors.Op = "mem.Index"
 	i.mu.Lock()
+	defer i.mu.Unlock()
+	for _, l := range i.lines {
+		if l.Path == mod && l.Version == ver {
+			return errors.E(op, fmt.Sprintf("%s@%s already indexed", mod, ver), errors.KindAlreadyExists)
+		}
+	}
 	i.lines = append(i.lines, &index.Line{
 		Path:      mod,
 		Version:   ver,
 		Timestamp: time.Now(),
 	})
-	i.mu.Unlock()
 	return nil
 }
 
diff --git a/pkg/index/mysql/mysql.go b/pkg/index/mysql/mysql.go
index 1ca85956..7298cd3f 100644
--- a/pkg/index/mysql/mysql.go
+++ b/pkg/index/mysql/mysql.go
@@ -12,6 +12,9 @@ import (
 	"github.com/gomods/athens/pkg/index"
 )
 
+// New returns a new Indexer with a MySQL implementation.
+// It attempts to connect to the DB and create the index table
+// if it doesn ot already exist.
 func New(cfg *config.MySQL) (index.Indexer, error) {
 	dataSource := getMySQLSource(cfg)
 	db, err := sql.Open("mysql", dataSource)
@@ -65,7 +68,7 @@ func (i *indexer) Index(ctx context.Context, mod, ver string) error {
 		time.Now().Format(time.RFC3339Nano),
 	)
 	if err != nil {
-		return errors.E(op, err)
+		return errors.E(op, err, getKind(err))
 	}
 	return nil
 }
@@ -103,3 +106,15 @@ func getMySQLSource(cfg *config.MySQL) string {
 	c.Params = cfg.Params
 	return c.FormatDSN()
 }
+
+func getKind(err error) int {
+	mysqlErr, ok := err.(*mysql.MySQLError)
+	if !ok {
+		return errors.KindUnexpected
+	}
+	switch mysqlErr.Number {
+	case 1062:
+		return errors.KindAlreadyExists
+	}
+	return errors.KindUnexpected
+}
diff --git a/pkg/index/postgres/postgres.go b/pkg/index/postgres/postgres.go
index 2f3c0b5a..6fb14c40 100644
--- a/pkg/index/postgres/postgres.go
+++ b/pkg/index/postgres/postgres.go
@@ -8,13 +8,16 @@ import (
 	"time"
 
 	// register the driver with database/sql
-	_ "github.com/lib/pq"
+	"github.com/lib/pq"
 
 	"github.com/gomods/athens/pkg/config"
 	"github.com/gomods/athens/pkg/errors"
 	"github.com/gomods/athens/pkg/index"
 )
 
+// New returns a new Indexer with a PostgreSQL implementation.
+// It attempts to connect to the DB and create the index table
+// if it doesn ot already exist.
 func New(cfg *config.Postgres) (index.Indexer, error) {
 	dataSource := getPostgresSource(cfg)
 	db, err := sql.Open("postgres", dataSource)
@@ -64,7 +67,7 @@ func (i *indexer) Index(ctx context.Context, mod, ver string) error {
 		time.Now().Format(time.RFC3339Nano),
 	)
 	if err != nil {
-		return errors.E(op, err)
+		return errors.E(op, err, getKind(err))
 	}
 	return nil
 }
@@ -104,3 +107,15 @@ func getPostgresSource(cfg *config.Postgres) string {
 	}
 	return strings.Join(args, " ")
 }
+
+func getKind(err error) int {
+	pqerr, ok := err.(*pq.Error)
+	if !ok {
+		return errors.KindUnexpected
+	}
+	switch pqerr.Code {
+	case "23505":
+		return errors.KindAlreadyExists
+	}
+	return errors.KindUnexpected
+}
diff --git a/pkg/stash/stasher.go b/pkg/stash/stasher.go
index 8d567cb5..1909ace4 100644
--- a/pkg/stash/stasher.go
+++ b/pkg/stash/stasher.go
@@ -74,7 +74,7 @@ func (s *stasher) Stash(ctx context.Context, mod, ver string) (string, error) {
 		return "", errors.E(op, err)
 	}
 	err = s.indexer.Index(ctx, mod, v.Semver)
-	if err != nil {
+	if err != nil && !errors.Is(err, errors.KindAlreadyExists) {
 		return "", errors.E(op, err)
 	}
 	return v.Semver, nil