diff --git a/headers/flat.hpp/detail/eq_compare.hpp b/headers/flat.hpp/detail/eq_compare.hpp
new file mode 100644
index 0000000..7498a41
--- /dev/null
+++ b/headers/flat.hpp/detail/eq_compare.hpp
@@ -0,0 +1,25 @@
+/*******************************************************************************
+ * This file is part of the "https://github.com/blackmatov/flat.hpp"
+ * For conditions of distribution and use, see copyright notice in LICENSE.md
+ * Copyright (C) 2019, by Matvey Cherevko (blackmatov@gmail.com)
+ ******************************************************************************/
+
+#pragma once
+
+namespace flat_hpp::detail
+{
+    template < typename Compare >
+    class eq_compare : public Compare {
+    public:
+        eq_compare() = default;
+
+        eq_compare(const Compare& compare)
+        : Compare(compare) {}
+
+        template < typename L, typename R >
+        bool operator()(const L& l, const R& r) const {
+            return !Compare::operator()(l, r)
+                && !Compare::operator()(r, l);
+        }
+    };
+}
diff --git a/headers/flat.hpp/flat_map.hpp b/headers/flat.hpp/flat_map.hpp
index fef0b96..0022533 100644
--- a/headers/flat.hpp/flat_map.hpp
+++ b/headers/flat.hpp/flat_map.hpp
@@ -15,6 +15,7 @@
 #include <type_traits>
 #include <initializer_list>
 
+#include "detail/eq_compare.hpp"
 #include "detail/pair_compare.hpp"
 #include "detail/is_transparent.hpp"
 
@@ -319,9 +320,13 @@ namespace flat_hpp
 
         template < typename InputIter >
         void insert(InputIter first, InputIter last) {
-            while ( first != last ) {
-                insert(*first++);
-            }
+            const auto mid_iter = data_.insert(data_.end(), first, last);
+            std::sort(mid_iter, data_.end(), value_comp());
+            std::inplace_merge(data_.begin(), mid_iter, data_.end());
+            data_.erase(
+                std::unique(data_.begin(), data_.end(),
+                    detail::eq_compare<value_compare>(value_comp())),
+                data_.end());
         }
 
         void insert(std::initializer_list<value_type> ilist) {
diff --git a/headers/flat.hpp/flat_multimap.hpp b/headers/flat.hpp/flat_multimap.hpp
index 2977061..77e0549 100644
--- a/headers/flat.hpp/flat_multimap.hpp
+++ b/headers/flat.hpp/flat_multimap.hpp
@@ -15,6 +15,7 @@
 #include <type_traits>
 #include <initializer_list>
 
+#include "detail/eq_compare.hpp"
 #include "detail/pair_compare.hpp"
 #include "detail/is_transparent.hpp"
 
@@ -315,9 +316,9 @@ namespace flat_hpp
 
         template < typename InputIter >
         void insert(InputIter first, InputIter last) {
-            while ( first != last ) {
-                insert(*first++);
-            }
+            const auto mid_iter = data_.insert(data_.end(), first, last);
+            std::sort(mid_iter, data_.end(), value_comp());
+            std::inplace_merge(data_.begin(), mid_iter, data_.end());
         }
 
         void insert(std::initializer_list<value_type> ilist) {
diff --git a/headers/flat.hpp/flat_multiset.hpp b/headers/flat.hpp/flat_multiset.hpp
index 52f2531..577b9d0 100644
--- a/headers/flat.hpp/flat_multiset.hpp
+++ b/headers/flat.hpp/flat_multiset.hpp
@@ -15,6 +15,7 @@
 #include <type_traits>
 #include <initializer_list>
 
+#include "detail/eq_compare.hpp"
 #include "detail/is_transparent.hpp"
 
 namespace flat_hpp
@@ -242,9 +243,9 @@ namespace flat_hpp
 
         template < typename InputIter >
         void insert(InputIter first, InputIter last) {
-            while ( first != last ) {
-                insert(*first++);
-            }
+            const auto mid_iter = data_.insert(data_.end(), first, last);
+            std::sort(mid_iter, data_.end(), value_comp());
+            std::inplace_merge(data_.begin(), mid_iter, data_.end());
         }
 
         void insert(std::initializer_list<value_type> ilist) {
diff --git a/headers/flat.hpp/flat_set.hpp b/headers/flat.hpp/flat_set.hpp
index aacd2e9..7d0f27c 100644
--- a/headers/flat.hpp/flat_set.hpp
+++ b/headers/flat.hpp/flat_set.hpp
@@ -15,6 +15,7 @@
 #include <type_traits>
 #include <initializer_list>
 
+#include "detail/eq_compare.hpp"
 #include "detail/is_transparent.hpp"
 
 namespace flat_hpp
@@ -246,9 +247,13 @@ namespace flat_hpp
 
         template < typename InputIter >
         void insert(InputIter first, InputIter last) {
-            while ( first != last ) {
-                insert(*first++);
-            }
+            const auto mid_iter = data_.insert(data_.end(), first, last);
+            std::sort(mid_iter, data_.end(), value_comp());
+            std::inplace_merge(data_.begin(), mid_iter, data_.end());
+            data_.erase(
+                std::unique(data_.begin(), data_.end(),
+                    detail::eq_compare<value_compare>(value_comp())),
+                data_.end());
         }
 
         void insert(std::initializer_list<value_type> ilist) {
diff --git a/untests/flat_map_tests.cpp b/untests/flat_map_tests.cpp
index b0c2c6c..72772c4 100644
--- a/untests/flat_map_tests.cpp
+++ b/untests/flat_map_tests.cpp
@@ -342,6 +342,14 @@ TEST_CASE("flat_map") {
             REQUIRE(i6 == s0.end() - 1);
             REQUIRE(s0 == map_t{{0,21},{1,42},{2,42},{3,84},{4,84},{5,100500},{6,100500}});
         }
+
+        {
+            map_t s0;
+            s0.insert({{6,2},{4,6},{2,4},{4,2}});
+            REQUIRE(s0 == map_t{{2,4},{4,6},{6,2}});
+            s0.insert({{9,3},{7,5},{3,9},{5,3},{5,3}});
+            REQUIRE(s0 == map_t{{2,4},{3,9},{4,6},{5,3},{6,2},{7,5},{9,3}});
+        }
     }
     SECTION("erasers") {
         using map_t = flat_map<int, unsigned>;
diff --git a/untests/flat_multimap_tests.cpp b/untests/flat_multimap_tests.cpp
index d7f9dbe..6ba52b7 100644
--- a/untests/flat_multimap_tests.cpp
+++ b/untests/flat_multimap_tests.cpp
@@ -342,6 +342,14 @@ TEST_CASE("flat_multimap") {
             REQUIRE(i6 == s0.end() - 1);
             REQUIRE(s0 == map_t{{0,21},{1,42},{1,21},{2,42},{3,84},{4,84},{5,100500},{6,100500}});
         }
+
+        {
+            map_t s0;
+            s0.insert({{6,2},{4,6},{2,4},{4,2}});
+            REQUIRE(s0 == map_t{{2,4},{4,6},{4,2},{6,2}});
+            s0.insert({{9,3},{7,5},{3,9},{5,3},{5,3}});
+            REQUIRE(s0 == map_t{{2,4},{3,9},{4,6},{4,2},{5,3},{5,3},{6,2},{7,5},{9,3}});
+        }
     }
     SECTION("erasers") {
         using map_t = flat_multimap<int, unsigned>;
diff --git a/untests/flat_multiset_tests.cpp b/untests/flat_multiset_tests.cpp
index 8864239..bee3325 100644
--- a/untests/flat_multiset_tests.cpp
+++ b/untests/flat_multiset_tests.cpp
@@ -318,6 +318,14 @@ TEST_CASE("flat_multiset") {
             s0.emplace_hint(s0.cend(), 6); // 1,2,3,3,4,5,6
             REQUIRE(s0 == set_t{1,2,3,3,4,5,6});
         }
+
+        {
+            set_t s0;
+            s0.insert({6,4,2,4});
+            REQUIRE(s0 == set_t{2,4,4,6});
+            s0.insert({9,7,3,5,5});
+            REQUIRE(s0 == set_t{2,3,4,4,5,5,6,7,9});
+        }
     }
     SECTION("erasers") {
         using set_t = flat_multiset<int>;
diff --git a/untests/flat_set_tests.cpp b/untests/flat_set_tests.cpp
index 91c59f1..ff1f939 100644
--- a/untests/flat_set_tests.cpp
+++ b/untests/flat_set_tests.cpp
@@ -318,6 +318,14 @@ TEST_CASE("flat_set") {
             s0.emplace_hint(s0.cend(), 6);
             REQUIRE(s0 == set_t{1,2,3,4,5,6});
         }
+
+        {
+            set_t s0;
+            s0.insert({6,4,2,4});
+            REQUIRE(s0 == set_t{2,4,6});
+            s0.insert({9,7,3,5,5});
+            REQUIRE(s0 == set_t{2,3,4,5,6,7,9});
+        }
     }
     SECTION("erasers") {
         using set_t = flat_set<int>;