diff --git a/changelog.d/791-sheet-built-twice.fixed.md b/changelog.d/791-sheet-built-twice.fixed.md
new file mode 100644
index 000000000..d6c5d9a37
--- /dev/null
+++ b/changelog.d/791-sheet-built-twice.fixed.md
@@ -0,0 +1,4 @@
+The desktop front end no longer builds a class's sheet twice when the kernel answers its
+route's description twice, once for the description fetched ahead and once for the one
+asked as the setup opened; the second build replaced every widget on the sheet for
+nothing ([#791](https://github.com/LupoLab/solprop/issues/791)).
diff --git a/gui/src/solprop_gui/ui/document.py b/gui/src/solprop_gui/ui/document.py
index 77ac1eed7..7f06d029d 100644
--- a/gui/src/solprop_gui/ui/document.py
+++ b/gui/src/solprop_gui/ui/document.py
@@ -268,8 +268,11 @@ def on_reply(self, reply: Reply) -> None:
         adapt = self._adapt.pop(reply.request_id, None)
         if purpose.startswith("describe-") and reply.ok and isinstance(value, dict):
             route = str(value["route"])
-            self.descriptions[route] = value
-            self.described.emit(route)
+            # A route asked ahead and asked again as its setup opened can be
+            # answered twice; a second answer would rebuild the sheet for nothing.
+            if route not in self.descriptions:
+                self.descriptions[route] = value
+                self.described.emit(route)
         elif purpose == "example" and reply.ok and isinstance(value, str):
             model = RecipeModel(value)
             if adapt is not None:
diff --git a/gui/tests/test_ui_workflow.py b/gui/tests/test_ui_workflow.py
index cc93ffd06..d2c2e8463 100644
--- a/gui/tests/test_ui_workflow.py
+++ b/gui/tests/test_ui_workflow.py
@@ -11,10 +11,11 @@
 
 import numpy as np
 import pytest
-from PySide6.QtCore import QEvent, QSettings, QStandardPaths, Qt
+from PySide6.QtCore import QCoreApplication, QEvent, QSettings, QStandardPaths, Qt
 from PySide6.QtGui import QFontDatabase, QStandardItemModel
 from PySide6.QtWidgets import QBoxLayout, QLabel, QLineEdit, QWidget
 from pytestqt.qtbot import QtBot
+from shiboken6 import isValid
 from solprop.constants import C
 from solprop.output import open_file
 from solprop.service.messages import Event
@@ -1436,6 +1437,26 @@ def test_a_step_index_fibre_solves_its_mode_for_its_numbers(
     )
 
 
+def test_a_second_answer_to_a_routes_description_leaves_its_sheet_standing(
+    window: WorkflowWindow, qtbot: QtBot
+) -> None:
+    # A route asked ahead and asked again as its setup opened is answered twice when
+    # the first answer is through before the second question is sent (#791).
+    window.begin_class("gnlse")
+    sheet = window.sheet
+    qtbot.waitUntil(lambda: sheet.problem is GNLSE, timeout=WAIT_MS)
+    order = sheet.derived["soliton_order"]
+    model = window.document.model
+    assert model is not None
+    route = model.route
+    again = window.document._ask("describe", {"route": route}, f"describe-{route}")
+    qtbot.waitUntil(lambda: again not in window.document._pending, timeout=WAIT_MS)
+    QCoreApplication.sendPostedEvents(None, QEvent.Type.DeferredDelete)
+    assert sheet.derived["soliton_order"] is order
+    assert isValid(order)
+    qtbot.waitUntil(lambda: order.text() == "3", timeout=WAIT_MS)
+
+
 def test_a_graded_index_fibre_shows_its_profile_and_its_modes(
     window: WorkflowWindow, qtbot: QtBot, tmp_path: Path
 ) -> None: