diff --git a/src/PhpSpreadsheet/Spreadsheet.php b/src/PhpSpreadsheet/Spreadsheet.php
index 943db95cc..530a56da8 100644
--- a/src/PhpSpreadsheet/Spreadsheet.php
+++ b/src/PhpSpreadsheet/Spreadsheet.php
@@ -514,6 +514,16 @@ class Spreadsheet implements JsonSerializable
     public function createSheet(?int $sheetIndex = null): Worksheet
     {
         $newSheet = new Worksheet($this);
+        $title = $newSheet->getTitle();
+        if ($this->sheetNameExists($title)) {
+            $i = 1;
+            $newTitle = "$title $i";
+            while ($this->sheetNameExists($newTitle)) {
+                ++$i;
+                $newTitle = "$title $i";
+            }
+            $newSheet->setTitle($newTitle);
+        }
         $this->addSheet($newSheet, $sheetIndex);
 
         return $newSheet;
diff --git a/src/PhpSpreadsheet/Worksheet/Worksheet.php b/src/PhpSpreadsheet/Worksheet/Worksheet.php
index 546a6ffe5..a3ee4818b 100644
--- a/src/PhpSpreadsheet/Worksheet/Worksheet.php
+++ b/src/PhpSpreadsheet/Worksheet/Worksheet.php
@@ -321,6 +321,7 @@ class Worksheet
     {
         // Set parent and title
         $this->parent = $parent;
+        $this->hash = spl_object_id($this);
         $this->setTitle($title, false);
         // setTitle can change $pTitle
         $this->setCodeName($this->getTitle());
@@ -349,7 +350,6 @@ class Worksheet
         $this->autoFilter = new AutoFilter('', $this);
         // Table collection
         $this->tableCollection = new ArrayObject();
-        $this->hash = spl_object_id($this);
     }
 
     /**
@@ -869,7 +869,7 @@ class Worksheet
             // Syntax check
             self::checkSheetTitle($title);
 
-            if ($this->parent) {
+            if ($this->parent && $this->parent->getIndex($this, true) >= 0) {
                 // Is there already such sheet name?
                 if ($this->parent->sheetNameExists($title)) {
                     // Use name, but append with lowest possible integer
@@ -899,7 +899,7 @@ class Worksheet
         // Set title
         $this->title = $title;
 
-        if ($this->parent && $this->parent->getCalculationEngine()) {
+        if ($this->parent && $this->parent->getIndex($this, true) >= 0 && $this->parent->getCalculationEngine()) {
             // New title
             $newTitle = $this->getTitle();
             $this->parent->getCalculationEngine()
diff --git a/tests/PhpSpreadsheetTests/Worksheet/Issue641Test.php b/tests/PhpSpreadsheetTests/Worksheet/Issue641Test.php
new file mode 100644
index 000000000..bbb6c8bf0
--- /dev/null
+++ b/tests/PhpSpreadsheetTests/Worksheet/Issue641Test.php
@@ -0,0 +1,82 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpOffice\PhpSpreadsheetTests\Worksheet;
+
+use PhpOffice\PhpSpreadsheet\Spreadsheet;
+use PHPUnit\Framework\TestCase;
+
+class Issue641Test extends TestCase
+{
+    /**
+     * Problem cloning sheet referred to in formulas.
+     */
+    public function testIssue641(): void
+    {
+        $xlsx = new Spreadsheet();
+        $xlsx->removeSheetByIndex(0);
+        $availableWs = [];
+
+        $worksheet = $xlsx->createSheet();
+        $worksheet->setTitle('Condensed A');
+        $worksheet->getCell('A1')->setValue("=SUM('Detailed A'!A1:A10)");
+        $worksheet->getCell('A2')->setValue(mt_rand(1, 30));
+        $availableWs[] = 'Condensed A';
+
+        $worksheet = $xlsx->createSheet();
+        $worksheet->setTitle('Condensed B');
+        $worksheet->getCell('A1')->setValue("=SUM('Detailed B'!A1:A10)");
+        $worksheet->getCell('A2')->setValue(mt_rand(1, 30));
+        $availableWs[] = 'Condensed B';
+
+        // at this point the value in worksheet 'Condensed B' cell A1 is
+        // =SUM('Detailed B'!A1:A10)
+
+        // worksheet in question is cloned and totals are attached
+        $totalWs1 = clone $xlsx->getSheet($xlsx->getSheetCount() - 1);
+        $totalWs1->setTitle('Condensed Total');
+        $xlsx->addSheet($totalWs1);
+        $formula = '=';
+        foreach ($availableWs as $ws) {
+            $formula .= sprintf("+'%s'!A2", $ws);
+        }
+        $totalWs1->getCell('A1')->setValue("=SUM('Detailed Total'!A1:A10)");
+        $totalWs1->getCell('A2')->setValue($formula);
+
+        $availableWs = [];
+
+        $worksheet = $xlsx->createSheet();
+        $worksheet->setTitle('Detailed A');
+        for ($step = 1; $step <= 10; ++$step) {
+            $worksheet->getCell("A{$step}")->setValue(mt_rand(1, 30));
+        }
+        $availableWs[] = 'Detailed A';
+
+        $worksheet = $xlsx->createSheet();
+        $worksheet->setTitle('Detailed B');
+        for ($step = 1; $step <= 10; ++$step) {
+            $worksheet->getCell("A{$step}")->setValue(mt_rand(1, 30));
+        }
+        $availableWs[] = 'Detailed B';
+
+        $totalWs2 = clone $xlsx->getSheet($xlsx->getSheetCount() - 1);
+        $totalWs2->setTitle('Detailed Total');
+        $xlsx->addSheet($totalWs2);
+
+        for ($step = 1; $step <= 10; ++$step) {
+            $formula = '=';
+            foreach ($availableWs as $ws) {
+                $formula .= sprintf("+'%s'!A%s", $ws, $step);
+            }
+            $totalWs2->getCell("A{$step}")->setValue($formula);
+        }
+
+        self::assertSame("=SUM('Detailed A'!A1:A10)", $xlsx->getSheetByName('Condensed A')?->getCell('A1')?->getValue());
+        self::assertSame("=SUM('Detailed B'!A1:A10)", $xlsx->getSheetByName('Condensed B')?->getCell('A1')?->getValue());
+        self::assertSame("=SUM('Detailed Total'!A1:A10)", $xlsx->getSheetByName('Condensed Total')?->getCell('A1')?->getValue());
+        self::assertSame("=+'Detailed A'!A1+'Detailed B'!A1", $xlsx->getSheetByName('Detailed Total')?->getCell('A1')?->getValue());
+
+        $xlsx->disconnectWorksheets();
+    }
+}