diff --git a/lib/microwaveprop/radio.ex b/lib/microwaveprop/radio.ex
index 7d4d168d..dc6fbb3d 100644
--- a/lib/microwaveprop/radio.ex
+++ b/lib/microwaveprop/radio.ex
@@ -329,6 +329,8 @@ defmodule Microwaveprop.Radio do
     end
   end
 
+  @dedup_window_seconds 3600
+
   def create_contact(attrs) do
     changeset = Contact.submission_changeset(%Contact{}, attrs)
 
@@ -336,26 +338,63 @@ defmodule Microwaveprop.Radio do
       grid1 = Ecto.Changeset.get_change(changeset, :grid1)
       grid2 = Ecto.Changeset.get_change(changeset, :grid2)
 
-      case {Maidenhead.to_latlon(grid1), Maidenhead.to_latlon(grid2)} do
-        {{:ok, {lat1, lon1}}, {:ok, {lat2, lon2}}} ->
-          distance = lat1 |> haversine_km(lon1, lat2, lon2) |> round() |> Decimal.new()
+      if duplicate_contact_exists?(changeset) do
+        changeset =
+          Ecto.Changeset.add_error(changeset, :station1, "a matching contact already exists within the last hour")
 
-          changeset
-          |> Ecto.Changeset.put_change(:pos1, %{"lat" => lat1, "lon" => lon1})
-          |> Ecto.Changeset.put_change(:pos2, %{"lat" => lat2, "lon" => lon2})
-          |> Ecto.Changeset.put_change(:distance_km, distance)
-          |> Ecto.Changeset.put_change(:user_submitted, true)
-          |> Repo.insert()
+        {:error, %{changeset | action: :insert}}
+      else
+        case {Maidenhead.to_latlon(grid1), Maidenhead.to_latlon(grid2)} do
+          {{:ok, {lat1, lon1}}, {:ok, {lat2, lon2}}} ->
+            distance = lat1 |> haversine_km(lon1, lat2, lon2) |> round() |> Decimal.new()
 
-        _ ->
-          changeset = Ecto.Changeset.add_error(changeset, :grid1, "could not resolve grid to coordinates")
-          {:error, %{changeset | action: :insert}}
+            changeset
+            |> Ecto.Changeset.put_change(:pos1, %{"lat" => lat1, "lon" => lon1})
+            |> Ecto.Changeset.put_change(:pos2, %{"lat" => lat2, "lon" => lon2})
+            |> Ecto.Changeset.put_change(:distance_km, distance)
+            |> Ecto.Changeset.put_change(:user_submitted, true)
+            |> Repo.insert()
+
+          _ ->
+            changeset = Ecto.Changeset.add_error(changeset, :grid1, "could not resolve grid to coordinates")
+            {:error, %{changeset | action: :insert}}
+        end
       end
     else
       {:error, %{changeset | action: :insert}}
     end
   end
 
+  defp duplicate_contact_exists?(changeset) do
+    s1 = Ecto.Changeset.get_field(changeset, :station1) |> to_string() |> String.upcase()
+    s2 = Ecto.Changeset.get_field(changeset, :station2) |> to_string() |> String.upcase()
+    g1 = Ecto.Changeset.get_field(changeset, :grid1) |> to_string() |> String.upcase()
+    g2 = Ecto.Changeset.get_field(changeset, :grid2) |> to_string() |> String.upcase()
+    band = Ecto.Changeset.get_field(changeset, :band)
+    ts = Ecto.Changeset.get_field(changeset, :qso_timestamp)
+
+    band_decimal = if is_binary(band), do: Decimal.new(band), else: band
+    min_ts = DateTime.add(ts, -@dedup_window_seconds, :second)
+    max_ts = DateTime.add(ts, @dedup_window_seconds, :second)
+
+    # Query for contacts with same band in the time window, then check
+    # station/grid pair match (order-independent) in Elixir
+    from(c in Contact,
+      where:
+        c.band == ^band_decimal and
+          c.qso_timestamp >= ^min_ts and
+          c.qso_timestamp <= ^max_ts and
+          c.flagged_invalid == false
+    )
+    |> Repo.all()
+    |> Enum.any?(fn c ->
+      a = {String.upcase(c.station1), String.upcase(c.grid1)}
+      b = {String.upcase(c.station2), String.upcase(c.grid2)}
+      pair = Enum.sort([a, b])
+      pair == Enum.sort([{s1, g1}, {s2, g2}])
+    end)
+  end
+
   # ── Contact Edits ───────────────────────────────────────────────
 
   @doc """
diff --git a/test/microwaveprop/radio/contact_submission_test.exs b/test/microwaveprop/radio/contact_submission_test.exs
index 5405244a..54db87b4 100644
--- a/test/microwaveprop/radio/contact_submission_test.exs
+++ b/test/microwaveprop/radio/contact_submission_test.exs
@@ -92,4 +92,40 @@ defmodule Microwaveprop.Radio.ContactSubmissionTest do
       refute Ecto.Changeset.get_change(changeset, :user_submitted)
     end
   end
+
+  describe "create_contact/1 deduplication" do
+    alias Microwaveprop.Radio
+
+    test "rejects duplicate contact within 1-hour window" do
+      assert {:ok, _contact} = Radio.create_contact(@valid_attrs)
+
+      assert {:error, changeset} = Radio.create_contact(@valid_attrs)
+      assert "a matching contact already exists within the last hour" in errors_on(changeset).station1
+    end
+
+    test "allows same stations on a different band" do
+      assert {:ok, _} = Radio.create_contact(@valid_attrs)
+      assert {:ok, _} = Radio.create_contact(%{@valid_attrs | band: "2304"})
+    end
+
+    test "allows same stations with different grids" do
+      assert {:ok, _} = Radio.create_contact(@valid_attrs)
+      assert {:ok, _} = Radio.create_contact(%{@valid_attrs | grid1: "EM13"})
+    end
+
+    test "allows contact outside the 1-hour window" do
+      assert {:ok, _} = Radio.create_contact(@valid_attrs)
+
+      later = DateTime.add(@valid_attrs.qso_timestamp, 3601, :second)
+      assert {:ok, _} = Radio.create_contact(%{@valid_attrs | qso_timestamp: later})
+    end
+
+    test "detects duplicates regardless of station order" do
+      assert {:ok, _} = Radio.create_contact(@valid_attrs)
+
+      swapped = %{@valid_attrs | station1: "K5TR", grid1: "EM00", station2: "W5XD", grid2: "EM12"}
+      assert {:error, changeset} = Radio.create_contact(swapped)
+      assert "a matching contact already exists within the last hour" in errors_on(changeset).station1
+    end
+  end
 end