diff --git a/authority/provisioner/provisioner.go b/authority/provisioner/provisioner.go
index 1ea48069..c994a5c2 100644
--- a/authority/provisioner/provisioner.go
+++ b/authority/provisioner/provisioner.go
@@ -370,13 +370,13 @@ func DefaultIdentityFunc(ctx context.Context, p Interface, email string, usernam
 // SanitizeStringSlices removes duplicated an empty strings.
 func SanitizeStringSlices(original []string) []string {
 	output := []string{}
-	seen := make(map[string]bool)
+	seen := make(map[string]struct{})
 	for _, entry := range original {
 		if entry == "" {
 			continue
 		}
 		if _, value := seen[entry]; !value {
-			seen[entry] = true
+			seen[entry] = struct{}{}
 			output = append(output, entry)
 		}
 	}
diff --git a/authority/provisioner/provisioner_test.go b/authority/provisioner/provisioner_test.go
index f1cf2ff5..267fb7d1 100644
--- a/authority/provisioner/provisioner_test.go
+++ b/authority/provisioner/provisioner_test.go
@@ -129,7 +129,7 @@ func TestDefaultIdentityFunc(t *testing.T) {
 			return test{
 				p:         &OIDC{},
 				email:     "john@smallstep.com",
-				usernames: []string{"johnny", "js", ""},
+				usernames: []string{"johnny", "js", "", "johnny", ""},
 				identity:  &Identity{Usernames: []string{"johnny", "js", "john", "john@smallstep.com"}},
 			}
 		},