diff --git a/charts/terdut-server/templates/deployment.yaml b/charts/terdut-server/templates/deployment.yaml
index e12ec2f..f2eb2b0 100644
--- a/charts/terdut-server/templates/deployment.yaml
+++ b/charts/terdut-server/templates/deployment.yaml
@@ -10,11 +10,14 @@ spec:
   selector:
     matchLabels:
       {{- include "terdut-server.selectorLabels" . | nindent 6 }}
-  # Recreate, not RollingUpdate, even though the PVC that forced it is gone: the
-  # sweeper, notifier and migration runner now take a Postgres advisory lock
-  # each, so two replicas overlapping during a rollout no longer both page for
-  # the same incident or race applying a migration, but new-incident creation
-  # on the first webhook for a brand-new groupKey is still unguarded.
+  # Recreate, not RollingUpdate, even though the PVC that forced it is gone:
+  # the sweeper, notifier and migration runner take a Postgres advisory lock
+  # each, and new-incident creation on the first webhook for a brand-new
+  # groupKey resolves its own insert conflict — so two replicas overlapping
+  # during a rollout no longer double-page, race a migration, or drop a
+  # webhook payload. Nothing left here actually requires Recreate anymore;
+  # it stays the default pending a deliberate decision to raise replicas
+  # above 1 and move to RollingUpdate.
   strategy:
     type: Recreate
   template:
diff --git a/internal/api/alertmanager.go b/internal/api/alertmanager.go
index 1badf72..012513f 100644
--- a/internal/api/alertmanager.go
+++ b/internal/api/alertmanager.go
@@ -376,6 +376,17 @@ func incidentForGroup(ctx context.Context, tx *sql.Tx, notify NotifyConfig, team
 // its own. Hence the querier rather than a *sql.Tx. A nil severity leaves the
 // column for refreshSeverity to fill from the member alerts; the sweeper passes
 // one because its incidents have no members to derive it from.
+//
+// Both callers get here only after their own SELECT found no open incident for
+// this group_key — but on more than one replica, two webhook deliveries for the
+// very first occurrence of a brand-new group_key can both pass that SELECT
+// before either INSERTs. ON CONFLICT DO NOTHING against
+// incidents_open_group_key_idx is what makes the loser's INSERT a no-op instead
+// of a unique-violation error that would otherwise roll back its entire
+// payload; existingOpenIncident then hands it the winner's row. Postgres
+// resolves that conflict only once the winner's transaction has committed (or
+// rolled back), so by the time this RETURNING comes back empty, the winner's
+// row is guaranteed visible to that follow-up SELECT.
 func openIncident(ctx context.Context, q querier, notify NotifyConfig, teamID int64, groupKey, title string, groupLabels map[string]string, severity *string) (int64, error) {
 	onCall, err := currentOnCall(ctx, q, teamID)
 	if err != nil {
@@ -391,10 +402,18 @@ func openIncident(ctx context.Context, q querier, notify NotifyConfig, teamID in
 	err = q.QueryRowContext(ctx, `
 		INSERT INTO incidents (team_id, group_key, title, group_labels, signature, status, severity, triggered_at, assigned_to)
 		VALUES ($1, $2, $3, $4::jsonb, $5, 'triggered', $6, $7, $8)
+		ON CONFLICT (team_id, group_key) WHERE resolved_at IS NULL DO NOTHING
 		RETURNING id`,
 		teamID, groupKey, title, string(labelsJSON), incidentSignature(groupLabels, title), severity,
 		time.Now().Unix(), onCall).Scan(&id)
-	if err != nil {
+	switch {
+	case err == sql.ErrNoRows:
+		// Lost the race: someone else's incident for this group_key exists now.
+		// Everything below — the trigger event, assignment, page, escalation
+		// clock — already happened for that row when it was created; attach to
+		// it rather than fail this call (and the whole payload) outright.
+		return existingOpenIncident(ctx, q, teamID, groupKey)
+	case err != nil:
 		return 0, err
 	}
 
@@ -423,6 +442,21 @@ func openIncident(ctx context.Context, q querier, notify NotifyConfig, teamID in
 	return id, nil
 }
 
+// existingOpenIncident looks up the open incident openIncident's own INSERT just
+// lost a conflict against — the same lookup incidentForGroup does before ever
+// calling openIncident, repeated here for the caller that arrived second.
+func existingOpenIncident(ctx context.Context, q querier, teamID int64, groupKey string) (int64, error) {
+	var id int64
+	err := q.QueryRowContext(ctx,
+		"SELECT id FROM incidents WHERE team_id = $1 AND group_key = $2 AND resolved_at IS NULL",
+		teamID, groupKey,
+	).Scan(&id)
+	if err != nil {
+		return 0, err
+	}
+	return id, nil
+}
+
 // linkAlert adds an alert to an incident, emitting a timeline entry only the
 // first time. Re-sends of an already-linked alert are silent.
 func linkAlert(ctx context.Context, tx *sql.Tx, incidentID, alertID int64) error {
diff --git a/internal/api/alertmanager_test.go b/internal/api/alertmanager_test.go
new file mode 100644
index 0000000..eceb4ab
--- /dev/null
+++ b/internal/api/alertmanager_test.go
@@ -0,0 +1,112 @@
+package api_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"sync"
+	"testing"
+)
+
+// TestWebhook_ConcurrentFirstOccurrenceOpensOneIncident reproduces two
+// replicas racing the very first webhook delivery for a brand-new group_key:
+// both see no open incident yet (incidentForGroup's own SELECT finds
+// nothing) and race openIncident's INSERT.
+//
+// The DB's own unique index already guarantees at most one incident either
+// way, with or without this fix — so "exactly one incident" alone cannot
+// tell a fixed run from a broken one. What ON CONFLICT handling actually
+// changes is what happens to the *loser*: before it, the loser's INSERT hit
+// incidents_open_group_key_idx's unique violation, which — since
+// upsertAlerts ran earlier in that same transaction — rolled back its whole
+// payload, alert insert included. ingest's error is only logged and
+// receiveWebhook answers 200 regardless, so nothing ever retried it: the
+// loser's alert silently never existed. That is the regression signal this
+// test checks — every caller's fingerprint must show up in /api/alerts, not
+// just the winner's.
+func TestWebhook_ConcurrentFirstOccurrenceOpensOneIncident(t *testing.T) {
+	s := newTS(t)
+
+	const callers = 8
+	const groupKey = "race-group"
+
+	// Every caller needs its own fingerprint. A shared one would serialize all
+	// of them at upsertAlerts' own ON CONFLICT (team_id, fingerprint) row lock,
+	// long before any of them reached incidentForGroup — which would hide the
+	// very race this test exists to force.
+	bodies := make([][]byte, callers)
+	for i := range callers {
+		payload := map[string]any{
+			"version":     "4",
+			"status":      "firing",
+			"groupKey":    groupKey,
+			"groupLabels": map[string]string{"alertname": "RaceAlert"},
+			"alerts": []map[string]any{amAlert(fmt.Sprintf("fp-race-%d", i), "RaceAlert", "firing",
+				"2026-05-20T10:00:00Z", "0001-01-01T00:00:00Z", nil)},
+		}
+		bodies[i], _ = json.Marshal(payload)
+	}
+
+	// A start line, so every request is fired as close to simultaneously as
+	// goroutine scheduling allows, rather than trickling out one dial at a
+	// time — the race window is the gap between incidentForGroup's SELECT and
+	// openIncident's INSERT, which a staggered start could easily miss.
+	var ready sync.WaitGroup
+	start := make(chan struct{})
+	statuses := make([]int, callers)
+	var wg sync.WaitGroup
+	for i := range callers {
+		ready.Add(1)
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			ready.Done()
+			<-start
+			resp, err := http.Post(s.URL+"/api/integrations/"+s.ingestKey+"/alertmanager",
+				"application/json", bytes.NewReader(bodies[i]))
+			if err != nil {
+				t.Errorf("post webhook #%d: %v", i, err)
+				return
+			}
+			defer resp.Body.Close()
+			statuses[i] = resp.StatusCode
+		}(i)
+	}
+	ready.Wait()
+	close(start)
+	wg.Wait()
+
+	for i, code := range statuses {
+		if code != http.StatusOK {
+			t.Errorf("webhook #%d returned %d, want 200", i, code)
+		}
+	}
+
+	var matched []any
+	for _, inc := range listIncidents(t, s, "") {
+		if inc["group_key"] == groupKey {
+			matched = append(matched, inc["id"])
+		}
+	}
+	if len(matched) != 1 {
+		t.Fatalf("expected exactly 1 incident for group_key %q after %d concurrent deliveries, got %d: %v",
+			groupKey, callers, len(matched), matched)
+	}
+
+	var alerts []map[string]any
+	decode(t, s.req(t, http.MethodGet, "/api/alerts", nil), &alerts)
+	seen := map[string]bool{}
+	for _, a := range alerts {
+		if fp, ok := a["fingerprint"].(string); ok {
+			seen[fp] = true
+		}
+	}
+	for i := range callers {
+		fp := fmt.Sprintf("fp-race-%d", i)
+		if !seen[fp] {
+			t.Errorf("alert %q is missing: its delivery's whole payload was silently rolled back "+
+				"when it lost the race for the incident", fp)
+		}
+	}
+}