diff --git a/internal/handler/hosted_login.go b/internal/handler/hosted_login.go
index a8628e0..77183df 100644
--- a/internal/handler/hosted_login.go
+++ b/internal/handler/hosted_login.go
@@ -465,6 +465,26 @@ input:focus{outline:none;border-color:var(--gold);box-shadow:0 0 0 3px rgba(143,
   var hasError = document.querySelector('.error') !== null;
   var manualForm = document.getElementById('manual-form');
 
+  // One submit per page: a second Sign In while the first is still redirecting
+  // replaces it, and the client's one-time OIDC state is gone by the time the
+  // second code reaches it.
+  var loginForm = manualForm.querySelector('form');
+  var loginButton = loginForm.querySelector('button[type="submit"]');
+  var loginLabel = loginButton.textContent;
+  loginForm.addEventListener('submit', function(e) {
+    if (loginForm.dataset.submitted) { e.preventDefault(); return; }
+    loginForm.dataset.submitted = '1';
+    loginButton.disabled = true;
+    loginButton.textContent = 'Signing in...';
+  });
+  // Back/forward cache restores the page as it was left; let it sign in again.
+  window.addEventListener('pageshow', function(e) {
+    if (!e.persisted) return;
+    delete loginForm.dataset.submitted;
+    loginButton.disabled = false;
+    loginButton.textContent = loginLabel;
+  });
+
   function showManualOnly() {
     manualForm.classList.add('show');
     manualForm.style.borderTop = 'none';
diff --git a/internal/handler/login_submit_once_test.go b/internal/handler/login_submit_once_test.go
new file mode 100644
index 0000000..3f52330
--- /dev/null
+++ b/internal/handler/login_submit_once_test.go
@@ -0,0 +1,53 @@
+package handler
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+)
+
+// TestLoginPagesSubmitOnce pins the one-submit guard on both credential forms.
+// A second Sign In while the first is still redirecting replaces the first
+// navigation; the relying party has already spent its one-time state on the
+// first code, so the second callback fails (OpenProject answers a bare 401).
+// The guard is browser-side, so this checks it is wired into each page.
+func TestLoginPagesSubmitOnce(t *testing.T) {
+	h, _ := testSetup(t)
+
+	const cb = "https://once.example/cb"
+	w := doJSON(h, "POST", "/api/admin/apps", map[string]interface{}{
+		"app_id": "once", "audience": "once", "redirect_uris": []string{cb},
+	}, adminHeaders())
+	if w.Code != http.StatusCreated {
+		t.Fatalf("create app: %d %s", w.Code, w.Body.String())
+	}
+
+	authorize := url.Values{}
+	authorize.Set("client_id", "once")
+	authorize.Set("redirect_uri", cb)
+	authorize.Set("response_type", "code")
+
+	pages := map[string]string{
+		"hosted /login":  "/login?manual=1",
+		"OIDC authorize": "/realms/test-issuer/protocol/openid-connect/auth?" + authorize.Encode(),
+	}
+	for name, path := range pages {
+		rec := httptest.NewRecorder()
+		h.ServeHTTP(rec, httptest.NewRequest("GET", path, nil))
+		if rec.Code != http.StatusOK {
+			t.Fatalf("%s: %d %s", name, rec.Code, rec.Body.String())
+		}
+		body := rec.Body.String()
+		for _, want := range []string{
+			`if (loginForm.dataset.submitted) { e.preventDefault(); return; }`,
+			`loginButton.disabled = true;`,
+			`window.addEventListener('pageshow'`,
+		} {
+			if !strings.Contains(body, want) {
+				t.Errorf("%s: login form is missing the one-submit guard (%q)", name, want)
+			}
+		}
+	}
+}
diff --git a/internal/handler/oidc.go b/internal/handler/oidc.go
index 75d58c8..fa9d6f4 100644
--- a/internal/handler/oidc.go
+++ b/internal/handler/oidc.go
@@ -1320,6 +1320,26 @@ input:focus{outline:none;border-color:var(--gold);box-shadow:0 0 0 3px rgba(143,
   var hasError = document.querySelector('.error') !== null;
   var manualForm = document.getElementById('manual-form');
 
+  // One submit per page: a second Sign In while the first is still redirecting
+  // replaces it, and the client's one-time OIDC state is gone by the time the
+  // second code reaches it.
+  var loginForm = manualForm.querySelector('form');
+  var loginButton = loginForm.querySelector('button[type="submit"]');
+  var loginLabel = loginButton.textContent;
+  loginForm.addEventListener('submit', function(e) {
+    if (loginForm.dataset.submitted) { e.preventDefault(); return; }
+    loginForm.dataset.submitted = '1';
+    loginButton.disabled = true;
+    loginButton.textContent = 'Signing in...';
+  });
+  // Back/forward cache restores the page as it was left; let it sign in again.
+  window.addEventListener('pageshow', function(e) {
+    if (!e.persisted) return;
+    delete loginForm.dataset.submitted;
+    loginButton.disabled = false;
+    loginButton.textContent = loginLabel;
+  });
+
   function showManualOnly() {
     manualForm.classList.add('show');
     manualForm.style.borderTop = 'none';