diff --git a/artefact/lib/artefact/op.ex b/artefact/lib/artefact/op.ex
index 31eac86..78ddf7a 100644
--- a/artefact/lib/artefact/op.ex
+++ b/artefact/lib/artefact/op.ex
@@ -157,11 +157,16 @@ defmodule Artefact.Op do
       nodes_from_a =
         Enum.map(a1.graph.nodes, fn n -> Map.get(primary_updates, n.uuid, n) end)
 
+      rel_offset = length(a1.graph.relationships)
+
       rels_from_b =
-        Enum.map(a2.graph.relationships, fn rel ->
+        a2.graph.relationships
+        |> Enum.with_index(rel_offset)
+        |> Enum.map(fn {rel, i} ->
           %{
             rel
-            | from_id: Map.get(b_id_remap, rel.from_id, rel.from_id),
+            | id: "r#{i}",
+              from_id: Map.get(b_id_remap, rel.from_id, rel.from_id),
               to_id: Map.get(b_id_remap, rel.to_id, rel.to_id)
           }
         end)
diff --git a/artefact/test/artefact_test.exs b/artefact/test/artefact_test.exs
index fb954dc..db33102 100644
--- a/artefact/test/artefact_test.exs
+++ b/artefact/test/artefact_test.exs
@@ -556,6 +556,7 @@ defmodule ArtefactTest do
                 tag: :self_harmonise,
                 details: %{uuid: uuid}
               }} = Artefact.harmonise(a, a, bindings)
+
       assert uuid == a.uuid
     end
 
@@ -1309,6 +1310,34 @@ defmodule ArtefactTest do
         Artefact.combine!(heart, other)
       end
     end
+
+    test "combine!/2 produces unique relationship IDs" do
+      shared_uuid = "019d0000-0000-7000-8000-000000000099"
+
+      a =
+        Artefact.new!(
+          base_label: "Knowing",
+          nodes: [
+            shared: [labels: ["Node"], uuid: shared_uuid],
+            b: [labels: ["B"]]
+          ],
+          relationships: [[from: :shared, type: "KNOWS", to: :b]]
+        )
+
+      b =
+        Artefact.new!(
+          base_label: "Valuing",
+          nodes: [
+            shared: [labels: ["Node"], uuid: shared_uuid],
+            c: [labels: ["C"]]
+          ],
+          relationships: [[from: :shared, type: "LOVES", to: :c]]
+        )
+
+      combined = Artefact.combine!(a, b)
+      ids = Enum.map(combined.graph.relationships, & &1.id)
+      assert ids == Enum.uniq(ids), "duplicate relationship IDs after combine!/2"
+    end
   end
 
   describe "Artefact.graft/3 — happy path with OurShells fixture" do