diff --git a/ClosedXML/Excel/XLWorksheets.cs b/ClosedXML/Excel/XLWorksheets.cs
index ea22509..4d14cb7 100644
--- a/ClosedXML/Excel/XLWorksheets.cs
+++ b/ClosedXML/Excel/XLWorksheets.cs
@@ -98,7 +98,7 @@
         public IXLWorksheet Add(String sheetName)
         {
             var sheet = new XLWorksheet(sheetName, _workbook);
-            _worksheets.Add(sheetName, sheet);
+            Add(sheetName, sheet);
             sheet._position = _worksheets.Count + _workbook.UnsupportedSheets.Count;
             return sheet;
         }
@@ -108,11 +108,19 @@
             _worksheets.Values.Where(w => w._position >= position).ForEach(w => w._position += 1);
             _workbook.UnsupportedSheets.Where(w => w.Position >= position).ForEach(w => w.Position += 1);
             var sheet = new XLWorksheet(sheetName, _workbook);
-            _worksheets.Add(sheetName, sheet);
+            Add(sheetName, sheet);
             sheet._position = position;
             return sheet;
         }
 
+        private void Add(String sheetName, XLWorksheet sheet)
+        {
+            if (_worksheets.Any(ws => ws.Key.Equals(sheetName, StringComparison.OrdinalIgnoreCase)))
+                throw new ArgumentException(String.Format("A worksheet with the same name ({0}) has already been added.", sheetName), nameof(sheetName));
+
+            _worksheets.Add(sheetName, sheet);
+        }
+
         public void Delete(String sheetName)
         {
             Delete(_worksheets[sheetName].Position);
@@ -172,9 +180,12 @@
         {
             if (XLHelper.IsNullOrWhiteSpace(oldSheetName) || !_worksheets.ContainsKey(oldSheetName)) return;
 
+            if (_worksheets.Any(ws1 => ws1.Key.Equals(newSheetName, StringComparison.OrdinalIgnoreCase)))
+                throw new ArgumentException(String.Format("A worksheet with the same name ({0}) has already been added.", newSheetName), nameof(newSheetName));
+
             var ws = _worksheets[oldSheetName];
             _worksheets.Remove(oldSheetName);
-            _worksheets.Add(newSheetName, ws);
+            Add(newSheetName, ws);
         }
     }
 }
diff --git a/ClosedXML_Tests/Excel/Worksheets/XLWorksheetTests.cs b/ClosedXML_Tests/Excel/Worksheets/XLWorksheetTests.cs
index 38615bf..f679187 100644
--- a/ClosedXML_Tests/Excel/Worksheets/XLWorksheetTests.cs
+++ b/ClosedXML_Tests/Excel/Worksheets/XLWorksheetTests.cs
@@ -84,6 +84,21 @@
         }
 
         [Test]
+        public void AddingDuplicateSheetNameThrowsException()
+        {
+            using (var wb = new XLWorkbook())
+            {
+                IXLWorksheet ws;
+                ws = wb.AddWorksheet("Sheet1");
+
+                Assert.Throws<ArgumentException>(() => wb.AddWorksheet("Sheet1"));
+
+                //Sheet names are case insensitive
+                Assert.Throws<ArgumentException>(() => wb.AddWorksheet("sheet1"));
+            }
+        }
+
+        [Test]
         public void MergedRanges()
         {
             IXLWorksheet ws = new XLWorkbook().Worksheets.Add("Sheet1");
@@ -126,4 +141,4 @@
             }
         }
     }
-}
+}
\ No newline at end of file