diff --git a/classes.php b/classes.php
index a7cc40e..6bbe3b8 100644
--- a/classes.php
+++ b/classes.php
@@ -52,8 +52,8 @@
             $this->columns = array_fill(0, 9, array());
             $this->regions = array_fill(0, 9, array());
             for ($regionRowId = 0; $regionRowId < 3; $regionRowId++) {
-                for($regionColumnId = 0; $regionColumnId < 3; $regionColumnId++) {
-                    for ($rowId = 3*$regionRowId; $rowId < 3*($regionRowId+1); $rowId++) {
+                for ($rowId = 3*$regionRowId; $rowId < 3*($regionRowId+1); $rowId++) {
+                    for($regionColumnId = 0; $regionColumnId < 3; $regionColumnId++) {
                         for ($columnId = 3*$regionColumnId; $columnId < 3*($regionColumnId+1); $columnId++) {
                             $regionId = 3*$regionRowId + $regionColumnId;
                             $box = new Box($rowId, $columnId, $regionId);
@@ -73,14 +73,37 @@
                         $box->neighbourhood[] = $neighbour;
             }
         }
+
+        function import($gridStr) {
+            foreach ($this->boxes as $i => $box) { 
+                $box->value = $gridStr[$i];
+            }
+            forEach($this->boxes as $box) {
+                forEach($box->neighbourhood as $neighbour)
+                    array_unset_value($box->value, $neighbour->candidates);
+            }
+        }
+
+        function containsDuplicates() {
+            foreach(array_merge($this->rows, $this->columns, $this->regions) as $area) {
+                $unknownBoxes = array_filter($area, "isUnknown");
+                foreach($unknownBoxes as $box1) {
+                    foreach($unknownBoxes as $box2) {
+                        if (($box1 !== $box2) && ($box1->value == $box2->value)) {
+                            return true;
+                        }
+                    }
+                }
+                return false;
+            }
+        }
             
         function generate() {
             // Init with a shuffle row
             $values = array("1", "2", "3", "4", "5", "6", "7", "8", "9");
             shuffle($values);
-            forEach($values as $columnId => $value) {
-                $box = $this->rows[0][$columnId];
-                $box->value = $value;
+            forEach($this->rows[0] as $columnId => $box) {
+                $box->value = $values[$columnId];
                 forEach($box->neighbourhood as $neighbour)
                     array_unset_value($box->value, $neighbour->candidates);
             }
@@ -145,16 +168,9 @@
                 if ($randomized) shuffle($testBox->candidates);
                 $stop = null;
                 foreach($testBox->candidates as $testBox->value) {
-                    $correctGrid = true;
                     foreach(array_filter($testBox->neighbourhood, "isUnknown") as $neighbour)
                         $neighbour->candidateRemoved[] = array_unset_value($testBox->value, $neighbour->candidates);
-                    foreach(array_filter($testBox->neighbourhood, "isUnknown") as $neighbour) {
-                        if (count($neighbour->candidates) == 0) {
-                            $correctGrid = false;
-                            break;
-                        }
-                    }
-                    if ($correctGrid) {
+                    if ($this->candidatesOnEachUnknownBoxeOf($testBox->neighbourhood)) {
                         $solutions = $this->solutionsGenerator($randomized);
                         foreach($solutions as $solution) {
                             $stop = (yield $solution);
@@ -171,9 +187,32 @@
                 }
                 $testBox->value = UNKNOWN;
             } else {
+                foreach(array($this->rows, $this->columns, $this->regions) as $areas) {
+                    foreach ($areas as $area) {
+                        foreach($area as $box1) {
+                            if (($box1->value == UNKNOWN) && (count($box1->candidates) == 0)) {
+                                return;
+                            }
+                            foreach($area as $box2) {
+                                if (($box1 !== $box2) && ($box1->value != UNKNOWN) && ($box1->value == $box2->value)) {
+                                    return;
+                                }
+                            }
+                        }
+                    }
+                }
                 yield $this->toString();
             }
         }
+
+        function candidatesOnEachUnknownBoxeOf($area) {
+            foreach($area as $box) {
+                if (($box->value == UNKNOWN) && (count($box->candidates) == 0)) {
+                    return false;
+                }
+            }
+            return true;
+        }
         
         function toString() {
             $str = "";
diff --git a/index.php b/index.php
index 162cc99..57d5214 100644
--- a/index.php
+++ b/index.php
@@ -3,6 +3,8 @@
     session_start();
     $grid = new Grid();
     $grid->generate();
-    header("Location: ".$_SERVER["REQUEST_SCHEME"]."://".$_SERVER["HTTP_HOST"].dirname($_SERVER["DOCUMENT_URI"])."/".$grid->toString());
+    $currentGrid = $grid->toString();
+    $_SESSION[$currentGrid] = "checked";
+    header("Location: ".$_SERVER["REQUEST_SCHEME"]."://".$_SERVER["HTTP_HOST"].dirname($_SERVER["DOCUMENT_URI"])."/".$currentGrid);
     exit();
 ?>
diff --git a/manifest.json.php b/manifest.json.php
index 12385e9..33cacb8 100644
--- a/manifest.json.php
+++ b/manifest.json.php
@@ -1,6 +1,9 @@
 <?php
     session_start();
-    $currentGrid = $_SESSION["currentGrid"];
+    if ($_SESSION["currentGrid"])
+        $currentGrid = $_SESSION["currentGrid"];
+    else
+        $currentGrid = ".";
 ?>
 {
     "short_name": "Sudoku",
diff --git a/service-worker.js.php b/service-worker.js.php
index 4198d9a..781afdc 100644
--- a/service-worker.js.php
+++ b/service-worker.js.php
@@ -1,6 +1,9 @@
 <?php
     session_start();
-    $currentGrid = $_SESSION["currentGrid"];
+    if ($_SESSION["currentGrid"])
+        $currentGrid = $_SESSION["currentGrid"];
+    else
+        $currentGrid = ".";
     header ("Content-type: application/javascript");
 ?>
 /*
diff --git a/sudoku.php b/sudoku.php
index 7ffd66e..e529a31 100644
--- a/sudoku.php
+++ b/sudoku.php
@@ -3,6 +3,24 @@
     session_start();
     $currentGrid = strip_tags($_GET['grid']);
     $_SESSION["currentGrid"] = $currentGrid;
+
+    if (!isset($_SESSION[$currentGrid])) {
+        $grid = new Grid();
+        $grid->import($currentGrid);
+        if ($grid->containsDuplicates()) {
+            $warning = "Cette grille contient des doublons.";
+        } else {
+            switch($grid->countSolutions(2)) {
+                case 0:
+                    $warning = "Cette grille n'a pas de solution.";
+                    break;
+                case 1:
+                    break;
+                default:
+                    $warning = "Cette grille a plusieurs solutions.";
+            }
+        }
+    }
 ?>
 <!DOCTYPE html>
 <html lang='fr' prefix="og: https://ogp.me/ns#">
@@ -95,7 +113,12 @@
             </div>
         </section>
         <section>
-            Remplissez la grille de sorte que chaque ligne, colonne et région (carré de 3×3 cases) contienne tous les chiffres de 1 à 9.
+<?php
+    if (isset($warning))
+        echo("            <strong>⚠️ $warning</strong><br/>\n");
+    else
+        echo("            Remplissez la grille de sorte que chaque ligne, colonne et région (carré de 3×3 cases) contienne tous les chiffres de 1 à 9.\n")
+?>
         </section>
         <ul id="contextMenu" class="context-menu"></ul>
         <footer>
diff --git a/thumbnail.png.php b/thumbnail.png.php
index 2a7fb49..912e416 100644
--- a/thumbnail.png.php
+++ b/thumbnail.png.php
@@ -1,9 +1,13 @@
 <?php
     require("classes.php");
     session_start();
-    $currentGrid = $_SESSION["currentGrid"];
+    if ($_SESSION["currentGrid"])
+        $currentGrid = $_SESSION["currentGrid"];
+    else
+        $currentGrid = ".528.3....4.9.1...39.562......73.129...1.64.7...42.3656.13.5...28.6.4...4.5287...w";
     header ("Content-type: image/png");
     $size = (int) $_GET['size'];
+
     $thumbnail = imagecreate($size, $size);
     $transparent = imagecolorallocate($thumbnail, 1, 1, 1);
     imagecolortransparent($thumbnail, $transparent);