diff --git a/pkg/storage/s3/checker.go b/pkg/storage/s3/checker.go
index 2851baee..6fee5a03 100644
--- a/pkg/storage/s3/checker.go
+++ b/pkg/storage/s3/checker.go
@@ -8,6 +8,7 @@ import (
 	"github.com/aws/aws-sdk-go/service/s3"
 	"github.com/gomods/athens/pkg/config"
 	"github.com/gomods/athens/pkg/errors"
+	"github.com/gomods/athens/pkg/log"
 	"github.com/gomods/athens/pkg/observ"
 )
 
@@ -22,25 +23,24 @@ func (s *Storage) Exists(ctx context.Context, module, version string) (bool, err
 		Bucket: aws.String(s.bucket),
 		Prefix: aws.String(fmt.Sprintf("%s/@v", module)),
 	}
-	var count int
+	found := make(map[string]struct{}, 3)
 	err := s.s3API.ListObjectsPagesWithContext(ctx, lsParams, func(loo *s3.ListObjectsOutput, lastPage bool) bool {
 		for _, o := range loo.Contents {
-			// sane assumption: no duplicate keys.
-			switch *o.Key {
-			case config.PackageVersionedName(module, version, "info"):
-				count++
-			case config.PackageVersionedName(module, version, "mod"):
-				count++
-			case config.PackageVersionedName(module, version, "zip"):
-				count++
+			if _, exists := found[*o.Key]; exists {
+				log.EntryFromContext(ctx).Warnf("duplicate key in prefix %q: %q", *lsParams.Prefix, *o.Key)
+				continue
+			}
+			if *o.Key == config.PackageVersionedName(module, version, "info") ||
+				*o.Key == config.PackageVersionedName(module, version, "mod") ||
+				*o.Key == config.PackageVersionedName(module, version, "zip") {
+				found[*o.Key] = struct{}{}
 			}
 		}
-		return count != 3
+		return len(found) < 3
 	})
 
 	if err != nil {
 		return false, errors.E(op, err, errors.M(module), errors.V(version))
 	}
-
-	return count == 3, nil
+	return len(found) == 3, nil
 }