diff --git a/conformance/gates.sh b/conformance/gates.sh
index c36a449..6208cce 100755
--- a/conformance/gates.sh
+++ b/conformance/gates.sh
@@ -517,6 +517,8 @@ self_test() {
     "$(gate_no_doubled_text "${testdata}/pass" && echo PASS || echo FAIL)"
   assert "G4 doubled text caught with the typed value redacted" FAIL \
     "$(gate_no_doubled_text "${testdata}/g4-doubled-text" && echo PASS || echo FAIL)"
+  assert "G4 doubling appended to existing content caught" FAIL \
+    "$(gate_no_doubled_text "${testdata}/g4-appended-doubling" && echo PASS || echo FAIL)"
   assert "G4 repeated character is not doubling" PASS \
     "$(gate_no_doubled_text "${testdata}/g4-repeated-character" && echo PASS || echo FAIL)"
   assert "G4 doubled text caught from the recorded value" FAIL \
diff --git a/conformance/testdata/g4-appended-doubling/exit_status b/conformance/testdata/g4-appended-doubling/exit_status
new file mode 100644
index 0000000..c227083
--- /dev/null
+++ b/conformance/testdata/g4-appended-doubling/exit_status
@@ -0,0 +1 @@
+0
\ No newline at end of file
diff --git a/conformance/testdata/g4-appended-doubling/output.log b/conformance/testdata/g4-appended-doubling/output.log
new file mode 100644
index 0000000..ca891a9
--- /dev/null
+++ b/conformance/testdata/g4-appended-doubling/output.log
@@ -0,0 +1 @@
+run complete
diff --git a/conformance/testdata/g4-appended-doubling/trace.jsonl b/conformance/testdata/g4-appended-doubling/trace.jsonl
new file mode 100644
index 0000000..7b3c183
--- /dev/null
+++ b/conformance/testdata/g4-appended-doubling/trace.jsonl
@@ -0,0 +1,2 @@
+{"step": 1, "timestamp": "2026-06-06T12:00:00.600000+05:30", "hierarchy": {"elements": [{"resourceId": "LoginScreen", "class": "android.view.View", "editable": true, "bounds": {"left": 34, "top": 87, "right": 286, "bottom": 135}, "attrs": {"resource-id": "LoginScreen", "class": "android.view.View", "text": ""}}, {"resourceId": "LoginEmail", "class": "android.widget.EditText", "editable": true, "bounds": {"left": 34, "top": 87, "right": 286, "bottom": 135}, "attrs": {"resource-id": "LoginEmail", "class": "android.widget.EditText", "text": "demo@"}}, {"resourceId": "LoginPassword", "class": "android.widget.EditText", "editable": true, "bounds": {"left": 34, "top": 87, "right": 286, "bottom": 135}, "attrs": {"resource-id": "LoginPassword", "class": "android.widget.EditText", "text": ""}}, {"resourceId": "LoginSubmit", "class": "android.view.View", "editable": true, "bounds": {"left": 34, "top": 87, "right": 286, "bottom": 135}, "attrs": {"resource-id": "LoginSubmit", "class": "android.view.View", "text": ""}}]}, "next_action": {"kind": "InputText", "text": "[redacted]", "selector": "testTag:LoginScreen > testTag:LoginEmail"}}
+{"step": 2, "timestamp": "2026-06-06T12:00:01.200000+05:30", "hierarchy": {"elements": [{"resourceId": "LoginScreen", "class": "android.view.View", "editable": true, "bounds": {"left": 34, "top": 87, "right": 286, "bottom": 135}, "attrs": {"resource-id": "LoginScreen", "class": "android.view.View", "text": ""}}, {"resourceId": "LoginEmail", "class": "android.widget.EditText", "editable": true, "bounds": {"left": 34, "top": 87, "right": 286, "bottom": 135}, "attrs": {"resource-id": "LoginEmail", "class": "android.widget.EditText", "text": "demo@folio.appfolio.app"}}, {"resourceId": "LoginPassword", "class": "android.widget.EditText", "editable": true, "bounds": {"left": 34, "top": 87, "right": 286, "bottom": 135}, "attrs": {"resource-id": "LoginPassword", "class": "android.widget.EditText", "text": ""}}, {"resourceId": "LoginSubmit", "class": "android.view.View", "editable": true, "bounds": {"left": 34, "top": 87, "right": 286, "bottom": 135}, "attrs": {"resource-id": "LoginSubmit", "class": "android.view.View", "text": ""}}]}}
diff --git a/conformance/testdata/generate.py b/conformance/testdata/generate.py
index b17a89c..98df24e 100644
--- a/conformance/testdata/generate.py
+++ b/conformance/testdata/generate.py
@@ -152,6 +152,16 @@ write_run(
     ],
 )
 
+# G4: the driver typed the value twice onto what the field already held, so the
+# whole value is not its own doubling. What it grew by is.
+write_run(
+    "g4-appended-doubling",
+    [
+        step(1, 600, login_screen("demo@", ""), input_action("LoginEmail")),
+        step(2, 600, login_screen("demo@folio.appfolio.app", "")),
+    ],
+)
+
 # G4: an action typed at coordinates names no field. The step carries nothing
 # the gate can check, and it may not stop the gate checking the rest.
 write_run(