diff --git a/sdks/go/pkg/beam/runners/prism/internal/handlerunner.go b/sdks/go/pkg/beam/runners/prism/internal/handlerunner.go
index a1eeeba02c4b..eecebde3d693 100644
--- a/sdks/go/pkg/beam/runners/prism/internal/handlerunner.go
+++ b/sdks/go/pkg/beam/runners/prism/internal/handlerunner.go
@@ -244,7 +244,7 @@ func (h *runner) ExecuteTransform(stageID, tid string, t *pipepb.PTransform, com
 		kc := coders[kcID]
 		ec := coders[ecID]
 
-		data = append(data, gbkBytes(ws, wc, kc, ec, inputData, coders, watermark))
+		data = append(data, gbkBytes(ws, wc, kc, ec, inputData, coders))
 		if len(data[0]) == 0 {
 			panic("no data for GBK")
 		}
@@ -290,7 +290,7 @@ func windowingStrategy(comps *pipepb.Components, tid string) *pipepb.WindowingSt
 }
 
 // gbkBytes re-encodes gbk inputs in a gbk result.
-func gbkBytes(ws *pipepb.WindowingStrategy, wc, kc, vc *pipepb.Coder, toAggregate [][]byte, coders map[string]*pipepb.Coder, watermark mtime.Time) []byte {
+func gbkBytes(ws *pipepb.WindowingStrategy, wc, kc, vc *pipepb.Coder, toAggregate [][]byte, coders map[string]*pipepb.Coder) []byte {
 	// Pick how the timestamp of the aggregated output is computed.
 	var outputTime func(typex.Window, mtime.Time, mtime.Time) mtime.Time
 	switch ws.GetOutputTime() {
@@ -333,9 +333,8 @@ func gbkBytes(ws *pipepb.WindowingStrategy, wc, kc, vc *pipepb.Coder, toAggregat
 	kd := pullDecoder(kc, coders)
 	vd := pullDecoder(vc, coders)
 
-	// Right, need to get the key coder, and the element coder.
-	// Cus I'll need to pull out anything the runner knows how to deal with.
-	// And repeat.
+	// Aggregate by windows and keys, using the window coder and KV coders.
+	// We need to extract and split the key bytes from the element bytes.
 	for _, data := range toAggregate {
 		// Parse out each element's data, and repeat.
 		buf := bytes.NewBuffer(data)
@@ -388,34 +387,41 @@ func gbkBytes(ws *pipepb.WindowingStrategy, wc, kc, vc *pipepb.Coder, toAggregat
 		}
 		// Use a decreasing sort (latest to earliest) so we can correct
 		// the output timestamp to the new end of window immeadiately.
-		// TODO need to correct this if output time is different.
 		sort.Slice(ordered, func(i, j int) bool {
 			return ordered[i].MaxTimestamp() > ordered[j].MaxTimestamp()
 		})
 
 		cur := ordered[0]
 		sessionData := windows[cur]
+		delete(windows, cur)
 		for _, iw := range ordered[1:] {
-			// If they overlap, then we merge the data.
+			// Check if the gap between windows is less than the gapSize.
+			// If not, this window is done, and we start a next window.
 			if iw.End+gapSize < cur.Start {
-				// Start a new session.
+				// Store current data with the current window.
 				windows[cur] = sessionData
+				// Use the incoming window instead, and clear it from the map.
 				cur = iw
 				sessionData = windows[iw]
+				delete(windows, cur)
+				// There's nothing to merge, since we've just started with this windowed data.
 				continue
 			}
-			// Extend the session
+			// Extend the session with the incoming window, and merge the the incoming window's data.
 			cur.Start = iw.Start
 			toMerge := windows[iw]
 			delete(windows, iw)
 			for k, kt := range toMerge {
 				skt := sessionData[k]
+				// Ensure the output time matches the given function.
+				skt.time = outputTime(cur, kt.time, skt.time)
 				skt.key = kt.key
 				skt.w = cur
 				skt.values = append(skt.values, kt.values...)
 				sessionData[k] = skt
 			}
 		}
+		windows[cur] = sessionData
 	}
 	// Everything's aggregated!
 	// Time to turn things into a windowed KV<K, Iterable<V>>
diff --git a/sdks/python/apache_beam/runners/portability/prism_runner_test.py b/sdks/python/apache_beam/runners/portability/prism_runner_test.py
index f1ccf66a2289..324fe5a17b54 100644
--- a/sdks/python/apache_beam/runners/portability/prism_runner_test.py
+++ b/sdks/python/apache_beam/runners/portability/prism_runner_test.py
@@ -40,7 +40,9 @@
 from apache_beam.runners.portability import portable_runner_test
 from apache_beam.testing.util import assert_that
 from apache_beam.testing.util import equal_to
+from apache_beam.transforms import window
 from apache_beam.transforms.sql import SqlTransform
+from apache_beam.utils import timestamp
 
 # Run as
 #
@@ -178,6 +180,26 @@ def create_options(self):
 
     return options
 
+  # Slightly more robust session window test:
+  # Validates that an inner grouping doesn't duplicate data either.
+  # Copied also because the timestamp in fn_runner_test.py isn't being
+  # inferred correctly as seconds for some reason, but as micros.
+  # The belabored specification is validating the timestamp type works at least.
+  # See https://github.com/apache/beam/issues/32085
+  def test_windowing(self):
+    with self.create_pipeline() as p:
+      res = (
+          p
+          | beam.Create([1, 2, 100, 101, 102, 123])
+          | beam.Map(
+              lambda t: window.TimestampedValue(
+                  ('k', t), timestamp.Timestamp.of(t).micros))
+          | beam.WindowInto(beam.transforms.window.Sessions(10))
+          | beam.GroupByKey()
+          | beam.Map(lambda k_vs1: (k_vs1[0], sorted(k_vs1[1]))))
+      assert_that(
+          res, equal_to([('k', [1, 2]), ('k', [100, 101, 102]), ('k', [123])]))
+
   # Can't read host files from within docker, read a "local" file there.
   def test_read(self):
     print('name:', __name__)