diff --git a/internal/calendar/calendar.go b/internal/calendar/calendar.go
index 829a9e0..cb62305 100644
--- a/internal/calendar/calendar.go
+++ b/internal/calendar/calendar.go
@@ -18,6 +18,7 @@ import (
 	"sort"
 	"strings"
 	"time"
+	"unicode"
 )
 
 // Fact sources. A calendar event reaches the store as a
@@ -153,14 +154,20 @@ func Overlapping(events []Event, from, to time.Time) []Event {
 	return out
 }
 
-// safeKey makes a summary safe to use inside a fact key (ASCII alphanumerics
-// and dashes). Non-Latin summaries collapse to their punctuation, which is why
-// the day prefix carries the identity and this only disambiguates within a day.
+// safeKey makes a summary safe to use inside a fact key: letters and digits in
+// any script, plus dashes, with space and underscore folded to a dash.
+//
+// It kept ASCII only until 04-08-2026, and dropped everything else. His
+// calendar is Russian, so "Встреча с Аней" and "Обед с мамой" both reduced to
+// "--" and produced the same key on the same day — the second event of the day
+// silently overwrote the first (Vikunja #443). Letting the letters through is
+// what makes the key identify the event. Migration #18 drops the keys written
+// under the old rule; they are re-derived on the next poll.
 func safeKey(s string) string {
 	var b strings.Builder
 	for _, r := range s {
 		switch {
-		case (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') || (r >= '0' && r <= '9') || r == '-':
+		case unicode.IsLetter(r) || unicode.IsDigit(r) || r == '-':
 			b.WriteRune(r)
 		case r == ' ' || r == '_':
 			b.WriteRune('-')
diff --git a/internal/calendar/calendar_test.go b/internal/calendar/calendar_test.go
index a7ffdbb..e701176 100644
--- a/internal/calendar/calendar_test.go
+++ b/internal/calendar/calendar_test.go
@@ -139,6 +139,9 @@ func TestSafeKey(t *testing.T) {
 		{"Hello_World", "Hello-World"},
 		{"special@#$chars!!", "specialchars"},
 		{"ALL_CAPS_123", "ALL-CAPS-123"},
+		// His calendar is Russian. These reduced to "--" and "--" (Vikunja #443).
+		{"Встреча с Аней", "Встреча-с-Аней"},
+		{"Обед с мамой", "Обед-с-мамой"},
 	}
 	for _, tt := range tests {
 		if got := safeKey(tt.in); got != tt.want {
@@ -263,3 +266,19 @@ func TestSourceTrust(t *testing.T) {
 		t.Errorf("Sources() = %v", Sources())
 	}
 }
+
+// Two Russian events on one day must not share a key. They did: safeKey kept
+// ASCII only, so both summaries collapsed to their spaces and the second event
+// overwrote the first in the store (Vikunja #443).
+func TestFactKeyDistinguishesRussianEventsOnOneDay(t *testing.T) {
+	day := time.Date(2026, 8, 4, 0, 0, 0, 0, time.UTC)
+	a := Event{Summary: "Встреча с Аней", Start: day.Add(10 * time.Hour), End: day.Add(11 * time.Hour)}
+	b := Event{Summary: "Обед с мамой", Start: day.Add(13 * time.Hour), End: day.Add(14 * time.Hour)}
+	if FactKeyIn(a, time.UTC) == FactKeyIn(b, time.UTC) {
+		t.Fatalf("both events keyed as %q", FactKeyIn(a, time.UTC))
+	}
+	// The day prefix still has to survive, because the store range-scans on it.
+	if !strings.HasPrefix(FactKeyIn(a, time.UTC), KeyPrefixForDay(day)) {
+		t.Fatalf("key %q lost the day prefix %q", FactKeyIn(a, time.UTC), KeyPrefixForDay(day))
+	}
+}
diff --git a/internal/store/migrations.go b/internal/store/migrations.go
index a266efe..8a94f64 100644
--- a/internal/store/migrations.go
+++ b/internal/store/migrations.go
@@ -208,6 +208,17 @@ ALTER TABLE reminders ADD COLUMN next_fire_ts INTEGER;`, // #2
 	// list_tasks into something that writes without the row changing by one
 	// byte. The fingerprint is the declared shape at approval time, so a
 	// redefinition is a re-approval instead of a silent upgrade.
+	`DELETE FROM facts
+	  WHERE key LIKE 'calendar_event_%'
+	    AND replace(substr(key, 25), '-', '') = '';`,
+	// #18 — drop the calendar keys written while safeKey dropped Cyrillic
+	// (Vikunja #443). Everything after the date prefix was punctuation, so
+	// every Russian event on one day shared one key and only the last one
+	// survived. Deleting rather than rewriting: a calendar fact is derived
+	// data, the next poll writes the day again under keys that identify the
+	// event, and the old rows would otherwise be recited as extra meetings.
+	// The filter is exact — it keeps any key whose summary part still has a
+	// letter or a digit in it.
 }
 
 // migrate applies every migration with a number greater than the DB's current
diff --git a/internal/store/migrations_test.go b/internal/store/migrations_test.go
index e561413..e0b2663 100644
--- a/internal/store/migrations_test.go
+++ b/internal/store/migrations_test.go
@@ -47,3 +47,36 @@ func TestMigrateAppliesOnceAndIsIdempotent(t *testing.T) {
 		t.Fatalf("after re-migrate user_version = %d, want %d", v, want)
 	}
 }
+
+// Migration #18 clears the calendar keys written while safeKey dropped
+// Cyrillic. Those rows are indistinguishable from real events on read, so
+// leaving them would recite one meeting as several (Vikunja #443).
+func TestCollapsedCalendarKeysAreDropped(t *testing.T) {
+	ctx := context.Background()
+	s := newTestStore(t)
+
+	rows := []string{
+		"calendar_event_20260804_--",             // "Встреча с Аней" under the old rule
+		"calendar_event_20260804_",               // a one-word Russian summary
+		"calendar_event_20260804_Встреча-с-Аней", // the new format
+		"calendar_event_20260804_Standup",        // an ASCII summary, always fine
+	}
+	for _, key := range rows {
+		if _, err := s.db.ExecContext(ctx,
+			`INSERT INTO facts (ts, kind, key, value, source, confidence) VALUES (0, 'env', ?, 'x', 'poll:caldav', 1.0)`,
+			key); err != nil {
+			t.Fatalf("seed %q: %v", key, err)
+		}
+	}
+	if _, err := s.db.ExecContext(ctx, migrations[17]); err != nil {
+		t.Fatalf("migration 18: %v", err)
+	}
+
+	var got int
+	if err := s.db.QueryRowContext(ctx, `SELECT count(*) FROM facts WHERE key LIKE 'calendar_event_%'`).Scan(&got); err != nil {
+		t.Fatal(err)
+	}
+	if got != 2 {
+		t.Fatalf("%d calendar rows left, want the 2 that identify their event", got)
+	}
+}