diff --git a/backend/app/services/relationship_service.py b/backend/app/services/relationship_service.py
index 0f3df4c..59229ff 100644
--- a/backend/app/services/relationship_service.py
+++ b/backend/app/services/relationship_service.py
@@ -4,7 +4,7 @@ Writes require editor rights; reads go through the privacy engine."""
 import uuid
 from datetime import UTC, datetime
 
-from sqlalchemy import or_, select
+from sqlalchemy import and_, or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.models.enums import ParentChildQualifier, RelationshipType
@@ -49,6 +49,38 @@ async def create_relationship(
         if not await _person_in_tree(session, pid, tree.id):
             raise NotFound("person not found in this tree")
 
+    # Reject an equivalent existing edge so the same two people can't be linked
+    # the same way twice. parent_child is directional (parent -> child);
+    # partnership/sibling are symmetric, so match the pair in either order.
+    if type is RelationshipType.parent_child:
+        pair = and_(
+            Relationship.person_from_id == person_from_id,
+            Relationship.person_to_id == person_to_id,
+        )
+    else:
+        pair = or_(
+            and_(
+                Relationship.person_from_id == person_from_id,
+                Relationship.person_to_id == person_to_id,
+            ),
+            and_(
+                Relationship.person_from_id == person_to_id,
+                Relationship.person_to_id == person_from_id,
+            ),
+        )
+    existing = (
+        await session.execute(
+            select(Relationship.id).where(
+                Relationship.tree_id == tree.id,
+                Relationship.type == type,
+                Relationship.deleted_at.is_(None),
+                pair,
+            )
+        )
+    ).scalar_one_or_none()
+    if existing is not None:
+        raise Conflict("these two people are already linked that way")
+
     relationship = Relationship(
         tree_id=tree.id,
         type=type,
diff --git a/backend/tests/test_relationship_dedup.py b/backend/tests/test_relationship_dedup.py
new file mode 100644
index 0000000..89101bc
--- /dev/null
+++ b/backend/tests/test_relationship_dedup.py
@@ -0,0 +1,65 @@
+"""Duplicate relationships are rejected (no double-linking)."""
+
+from tests.conftest import auth, register
+
+
+async def _setup(client, email):
+    h = auth(await register(client, email))
+    tid = (await client.post("/api/v1/trees", json={"name": "T"}, headers=h)).json()["id"]
+
+    async def person(given):
+        return (
+            await client.post(f"/api/v1/trees/{tid}/persons", json={"given": given}, headers=h)
+        ).json()["id"]
+
+    return h, tid, person
+
+
+async def test_duplicate_parent_child_rejected(client):
+    h, tid, person = await _setup(client, "dup-pc@example.com")
+    karl = await person("Karl")
+    kid = await person("Kid")
+    body = {"type": "parent_child", "person_from_id": karl, "person_to_id": kid}
+
+    first = await client.post(f"/api/v1/trees/{tid}/relationships", json=body, headers=h)
+    assert first.status_code == 201
+    dup = await client.post(f"/api/v1/trees/{tid}/relationships", json=body, headers=h)
+    assert dup.status_code == 409
+
+
+async def test_duplicate_partnership_either_direction_rejected(client):
+    h, tid, person = await _setup(client, "dup-sp@example.com")
+    a = await person("A")
+    b = await person("B")
+
+    first = await client.post(
+        f"/api/v1/trees/{tid}/relationships",
+        json={"type": "partnership", "person_from_id": a, "person_to_id": b},
+        headers=h,
+    )
+    assert first.status_code == 201
+    # Same couple, reversed order — still a duplicate.
+    dup = await client.post(
+        f"/api/v1/trees/{tid}/relationships",
+        json={"type": "partnership", "person_from_id": b, "person_to_id": a},
+        headers=h,
+    )
+    assert dup.status_code == 409
+
+
+async def test_reverse_parent_child_is_allowed(client):
+    """A->B as parent_child shouldn't block B->A (different meaning)."""
+    h, tid, person = await _setup(client, "dup-rev@example.com")
+    a = await person("A")
+    b = await person("B")
+    r1 = await client.post(
+        f"/api/v1/trees/{tid}/relationships",
+        json={"type": "parent_child", "person_from_id": a, "person_to_id": b},
+        headers=h,
+    )
+    r2 = await client.post(
+        f"/api/v1/trees/{tid}/relationships",
+        json={"type": "parent_child", "person_from_id": b, "person_to_id": a},
+        headers=h,
+    )
+    assert r1.status_code == 201 and r2.status_code == 201
diff --git a/frontend/app/trees/[id]/persons/[personId]/page.tsx b/frontend/app/trees/[id]/persons/[personId]/page.tsx
index 506c697..5e298c3 100644
--- a/frontend/app/trees/[id]/persons/[personId]/page.tsx
+++ b/frontend/app/trees/[id]/persons/[personId]/page.tsx
@@ -150,6 +150,7 @@ export default function PersonDetailPage() {
   const [relKind, setRelKind] = useState<"parent" | "child" | "partner" | "sibling">("parent");
   const [relOther, setRelOther] = useState("");
   const [relQual, setRelQual] = useState<Qualifier>("biological");
+  const [relErr, setRelErr] = useState<string | null>(null);
 
   // Add-name form + inline edit.
   const [nameType, setNameType] = useState("married");
@@ -363,9 +364,12 @@ export default function PersonDetailPage() {
   async function addRel(e: React.FormEvent) {
     e.preventDefault();
     if (!relOther) return;
+    setRelErr(null);
     if (await linkRelative(relOther)) {
       setRelOther("");
       load();
+    } else {
+      setRelErr("They're already linked that way.");
     }
   }
 
@@ -1142,6 +1146,7 @@ export default function PersonDetailPage() {
               <Button type="submit">Link</Button>
             </form>
           )}
+          {relErr && <p className="text-sm text-red-600">{relErr}</p>}
         </CardContent>
       </Card>
 
diff --git a/frontend/app/trees/[id]/tree/page.tsx b/frontend/app/trees/[id]/tree/page.tsx
index 3cdb672..73dbbc1 100644
--- a/frontend/app/trees/[id]/tree/page.tsx
+++ b/frontend/app/trees/[id]/tree/page.tsx
@@ -39,6 +39,7 @@ export default function TreePage() {
   const [status, setStatus] = useState<"loading" | "empty" | "ready" | "error">("loading");
   const [focusId, setFocusId] = useState<string | null>(null);
   const [mode, setMode] = useState<Mode>("landscape");
+  const [renderNote, setRenderNote] = useState<string | null>(null);
 
   useEffect(() => {
     let cancelled = false;
@@ -108,11 +109,46 @@ export default function TreePage() {
     if (status !== "ready" || mode === "fan" || !containerRef.current) return;
     let cancelled = false;
     (async () => {
-      // Only link to people that still exist — a soft-deleted person leaves
-      // dangling relationship rows, and family-chart breaks on an id with no
-      // matching datum. Filter them out so a deletion never blanks the tree.
+      // Sanitize the graph before handing it to family-chart, which recurses
+      // through parents and will blow the stack (blank tree) on a cycle — e.g. a
+      // person edited into being their own ancestor.
       const alive = new Set(people.map((pp) => pp.id));
-      const keep = (ids: string[]) => ids.filter((id) => alive.has(id));
+      const ok = (ids: string[], self: string) =>
+        [...new Set(ids)].filter((id) => alive.has(id) && id !== self);
+
+      // Build an acyclic set of parent edges: skip any edge that would make a
+      // person their own ancestor. Children are derived from the kept edges so
+      // parent/child stays consistent.
+      const parentsMap = new Map<string, string[]>();
+      const childrenMap = new Map<string, string[]>();
+      let dropped = 0;
+      const isAncestorOf = (ancestor: string, of: string): boolean => {
+        const stack = [...(parentsMap.get(of) ?? [])];
+        const seen = new Set<string>();
+        while (stack.length) {
+          const n = stack.pop()!;
+          if (n === ancestor) return true;
+          if (seen.has(n)) continue;
+          seen.add(n);
+          for (const p of parentsMap.get(n) ?? []) stack.push(p);
+        }
+        return false;
+      };
+      for (const pp of people) {
+        const accepted: string[] = [];
+        for (const par of ok(parentsOf(pp.id), pp.id)) {
+          // Edge "pp has parent par" loops if pp is already an ancestor of par.
+          if (isAncestorOf(pp.id, par)) {
+            dropped++;
+            continue;
+          }
+          accepted.push(par);
+          parentsMap.set(pp.id, accepted);
+          childrenMap.set(par, [...(childrenMap.get(par) ?? []), pp.id]);
+        }
+        parentsMap.set(pp.id, accepted);
+      }
+
       const data = people.map((pp) => {
         const [fn, ln] = splitName(pp.primary_name);
         return {
@@ -124,32 +160,47 @@ export default function TreePage() {
             gender: pp.gender === "female" ? "F" : "M",
           },
           rels: {
-            spouses: keep(partnersOf(pp.id)),
-            parents: keep(parentsOf(pp.id)),
-            children: keep(childrenOf(pp.id)),
+            spouses: ok(partnersOf(pp.id), pp.id),
+            parents: parentsMap.get(pp.id) ?? [],
+            children: childrenMap.get(pp.id) ?? [],
           },
         };
       });
       const f3 = await import("family-chart");
       if (cancelled || !containerRef.current) return;
-      containerRef.current.innerHTML = "";
-      const chart = f3.createChart(containerRef.current, data);
-      chart.setCardHtml().setCardDisplay([["first name", "last name"], ["birthday"]]);
-      if (mode === "portrait") chart.setOrientationVertical();
-      else chart.setOrientationHorizontal();
-      // Show enough generations that a recenter reveals grandparents + children.
-      chart.setAncestryDepth?.(3);
-      chart.setProgenyDepth?.(2);
-      // Default card click recenters the whole hourglass; sync focus for the
-      // "Open profile" link after every (re)build.
-      chart.setAfterUpdate?.(() => {
-        const md = chart.getMainDatum?.();
-        const id = md?.id ?? md?.data?.id;
-        if (id) setFocusId(id);
-      });
-      chartRef.current = chart;
-      if (focusId) chart.updateMainId(focusId);
-      chart.updateTree({ initial: true });
+      try {
+        containerRef.current.innerHTML = "";
+        const chart = f3.createChart(containerRef.current, data);
+        chart.setCardHtml().setCardDisplay([["first name", "last name"], ["birthday"]]);
+        if (mode === "portrait") chart.setOrientationVertical();
+        else chart.setOrientationHorizontal();
+        // Show enough generations that a recenter reveals grandparents + children.
+        chart.setAncestryDepth?.(3);
+        chart.setProgenyDepth?.(2);
+        // Default card click recenters the whole hourglass; sync focus for the
+        // "Open profile" link after every (re)build.
+        chart.setAfterUpdate?.(() => {
+          const md = chart.getMainDatum?.();
+          const id = md?.id ?? md?.data?.id;
+          if (id) setFocusId(id);
+        });
+        chartRef.current = chart;
+        if (focusId) chart.updateMainId(focusId);
+        chart.updateTree({ initial: true });
+        setRenderNote(
+          dropped > 0
+            ? `Skipped ${dropped} conflicting parent link${dropped === 1 ? "" : "s"} (a person can't be their own ancestor). Open the people involved to fix the relationship.`
+            : null,
+        );
+      } catch (err) {
+        // Never leave a blank canvas — show a message and let them fix via the
+        // Family view / person pages.
+        console.error("tree render failed", err);
+        if (containerRef.current) containerRef.current.innerHTML = "";
+        setRenderNote(
+          "The tree couldn't be drawn — a relationship may be conflicting. Use the Family view to open the affected people and check their parents/children.",
+        );
+      }
     })();
     return () => {
       cancelled = true;
@@ -242,6 +293,12 @@ export default function TreePage() {
       )}
       {status === "error" && <p className="text-[var(--muted)]">Could not render the tree.</p>}
 
+      {renderNote && mode !== "fan" && (
+        <p className="rounded-md border border-bronze/40 bg-bronze/[0.06] px-3 py-2 text-sm text-bronze">
+          {renderNote}
+        </p>
+      )}
+
       {status === "ready" && mode === "fan" && focusId ? (
         <div className="rounded-xl border border-[var(--border)] bg-[var(--surface)] p-4">
           <FanChart