diff --git a/MissionEditor/IniFile.h b/MissionEditor/IniFile.h
index 5a9ffe8..d1d576e 100644
--- a/MissionEditor/IniFile.h
+++ b/MissionEditor/IniFile.h
@@ -137,12 +137,13 @@ public:
 		return this->FindValue(val) >= 0;
 	}
 
-	size_t LowerBound(const CString& key) const {
+	// <pos, existed?>
+	std::pair<size_t, bool> LowerBound(const CString& key) const {
 		auto const it = value_pos.lower_bound(key);
 		if (it != value_pos.end()) {
-			return it->second;
+			return { it->second, it->first == key };
 		}
-		return value_pairs.size();
+		return { value_pairs.size(), false };
 	}
 
 	// ==================== Modify
@@ -176,7 +177,7 @@ public:
 		value_pairs.push_back({ key, value });
 		value_pos.insert_or_assign(key, value_pairs.size() - 1);
 	}
-
+	// not recommended to call it directly
 	void InsertAt(size_t idx, CString&& key, CString&& value) {
 		if (idx > value_pairs.size()) {
 			idx = value_pairs.size() - 1;
@@ -188,9 +189,13 @@ public:
 			it->second++;
 		}
 	}
-	void Insert(CString&& key, CString&& value) {
-		auto const pos = LowerBound(key);
-		InsertAt(pos, std::move(key), std::move(value));
+	void InsertOrAssign(CString&& key, CString&& value) {
+		auto const [pos, found] = LowerBound(key);
+		if (!found) {
+			InsertAt(pos, std::move(key), std::move(value));
+		}
+		// existed, assign
+		value_pairs[pos].second = std::move(value);
 	}
 
 	// ==================== Delete
diff --git a/MissionEditor/MapData.cpp b/MissionEditor/MapData.cpp
index 80c0d9f..6700458 100644
--- a/MissionEditor/MapData.cpp
+++ b/MissionEditor/MapData.cpp
@@ -2060,7 +2060,7 @@ BOOL CMapData::AddWaypoint(CString id, DWORD dwPos)
 		pSec = &m_mapfile.AddSection("Waypoints");
 	}
 
-	pSec->Insert(std::move(id), k);
+	pSec->InsertOrAssign(std::move(id), k);
 
 	if (!m_noAutoObjectUpdate) {
 		UpdateWaypoints(FALSE);
diff --git a/UnitTest/CIni_Test.cpp b/UnitTest/CIni_Test.cpp
index 1895cac..c2dfa04 100644
--- a/UnitTest/CIni_Test.cpp
+++ b/UnitTest/CIni_Test.cpp
@@ -260,15 +260,19 @@ TEST(CIniFileClass, IniLowerBoundInsertTest) {
 	auto const pSec = file.TryGetSection("Waypoints");
 	EXPECT_NE(pSec, nullptr);
 	auto const pos = pSec->LowerBound("4");
-	EXPECT_LE(pos, pSec->Size());
-	pSec->InsertAt(pos, "4", "432156");
+	EXPECT_LE(pos.first, pSec->Size());
+	// not existed
+	EXPECT_EQ(pos.second, false);
+	pSec->InsertAt(pos.first, "4", "432156");
 	EXPECT_EQ(432156, file.GetInteger("Waypoints", "4"));
 	EXPECT_EQ("432156", file["Waypoints"].Nth(3).second);
 	EXPECT_EQ("789654", file["Waypoints"].Nth(4).second);
-	pSec->Insert("9", "149367");
-	pSec->Insert("11", "987654");
+	pSec->InsertOrAssign("9", "149367");
+	pSec->InsertOrAssign("11", "987654");
+	pSec->InsertOrAssign("10", "159356"); // existed replace
 	EXPECT_EQ(149367, file.GetInteger("Waypoints", "9"));
+	EXPECT_EQ(159356, file.GetInteger("Waypoints", "10"));
 	EXPECT_EQ(987654, file.GetInteger("Waypoints", "11"));
 	EXPECT_EQ("987654", file["Waypoints"].Nth(pSec->Size() - 1).second);
-	EXPECT_EQ("159357", file["Waypoints"].Nth(pSec->Size() - 2).second);
+	EXPECT_EQ("159356", file["Waypoints"].Nth(pSec->Size() - 2).second);
 }
\ No newline at end of file