diff --git a/CHANGELOG.txt b/CHANGELOG.txt
index f518a1564a..ba3a2d8a61 100644
--- a/CHANGELOG.txt
+++ b/CHANGELOG.txt
@@ -33,6 +33,7 @@ Changelog
  * Fix: Prevent tab labels on page edit view from being cut off on very narrow screens (Kevin Howbrook)
  * Fix: Very long words in page listings are now broken where necessary (Kevin Howbrook)
  * Fix: Language chosen in user preferences no longer persists on subsequent requests (Bojan Mihelac)
+ * Fix: Prevent new block IDs from being assigned on repeated calls to `StreamBlock.get_prep_value` (Colin Klein)
 
 
 2.6.1 (05.08.2019)
diff --git a/CONTRIBUTORS.rst b/CONTRIBUTORS.rst
index 36f8a1fe17..0225806b28 100644
--- a/CONTRIBUTORS.rst
+++ b/CONTRIBUTORS.rst
@@ -395,6 +395,7 @@ Contributors
 * Sarath Kumar Somana
 * Dani Hodovic
 * Janne Alatalo
+* Colin Klein
 
 Translators
 ===========
diff --git a/docs/releases/2.7.rst b/docs/releases/2.7.rst
index 797f2882fd..93723d8b4c 100644
--- a/docs/releases/2.7.rst
+++ b/docs/releases/2.7.rst
@@ -57,6 +57,7 @@ Bug fixes
  * Prevent tab labels on page edit view from being cut off on very narrow screens (Kevin Howbrook)
  * Very long words in page listings are now broken where necessary (Kevin Howbrook)
  * Language chosen in user preferences no longer persists on subsequent requests (Bojan Mihelac)
+ * Prevent new block IDs from being assigned on repeated calls to ``StreamBlock.get_prep_value`` (Colin Klein)
 
 
 Upgrade considerations
diff --git a/wagtail/core/blocks/stream_block.py b/wagtail/core/blocks/stream_block.py
index 884fbb2029..e684dc9500 100644
--- a/wagtail/core/blocks/stream_block.py
+++ b/wagtail/core/blocks/stream_block.py
@@ -432,20 +432,22 @@ class StreamValue(Sequence):
                 # value (stream_data_item here) is still valid
                 prep_value_item = stream_data_item
 
+                # As this method is preparing this value to be saved to the database,
+                # this is an appropriate place to ensure that each block has a unique id.
+                prep_value_item['id'] = prep_value_item.get('id', str(uuid.uuid4()))
+
             else:
                 # convert the bound block back into JSONish data
                 child = self[i]
+                # As this method is preparing this value to be saved to the database,
+                # this is an appropriate place to ensure that each block has a unique id.
+                child.id = child.id or str(uuid.uuid4())
                 prep_value_item = {
                     'type': child.block.name,
                     'value': child.block.get_prep_value(child.value),
                     'id': child.id,
                 }
 
-            # As this method is preparing this value to be saved to the database,
-            # this is an appropriate place to ensure that each block has a unique id.
-            if not prep_value_item.get('id'):
-                prep_value_item['id'] = str(uuid.uuid4())
-
             prep_value.append(prep_value_item)
 
         return prep_value
diff --git a/wagtail/core/tests/test_blocks.py b/wagtail/core/tests/test_blocks.py
index 7ab2a507ac..c0f413c8e9 100644
--- a/wagtail/core/tests/test_blocks.py
+++ b/wagtail/core/tests/test_blocks.py
@@ -2748,6 +2748,12 @@ class TestStreamBlock(WagtailTestUtils, SimpleTestCase):
         # ID to this block, as it didn't have one already.
         self.assertTrue(jsonish_value[1]['id'])
 
+        # Calling get_prep_value again should preserve existing IDs, including the one
+        # just assigned to block 1
+        jsonish_value_again = block.get_prep_value(value)
+        self.assertEqual(jsonish_value[0]['id'], jsonish_value_again[0]['id'])
+        self.assertEqual(jsonish_value[1]['id'], jsonish_value_again[1]['id'])
+
     def test_get_prep_value_not_lazy(self):
         stream_data = [
             ('heading', 'this is my heading', '0000'),