diff --git a/.gitignore b/.gitignore
index b0fcac54..16423042 100644
--- a/.gitignore
+++ b/.gitignore
@@ -20,6 +20,7 @@ TAGS
 *.bak
 !test/functional/xlsx_files/*.xlsx
 *.pyc
+.pytest_cache/
 .cproject
 .project
 .pydevproject
diff --git a/src/styles.c b/src/styles.c
index 6caa9e26..cdd4a952 100644
--- a/src/styles.c
+++ b/src/styles.c
@@ -155,6 +155,7 @@ _write_num_fmts(lxw_styles *self)
     struct xml_attribute_list attributes;
     struct xml_attribute *attribute;
     lxw_format *format;
+    uint16_t last_format_index = 0;
 
     if (!self->num_format_count)
         return;
@@ -171,7 +172,13 @@ _write_num_fmts(lxw_styles *self)
         if (format->num_format_index < 164)
             continue;
 
+        /* Ignore duplicates which have an already used index. */
+        if (format->num_format_index <= last_format_index)
+            continue;
+
         _write_num_fmt(self, format->num_format_index, format->num_format);
+
+        last_format_index = format->num_format_index;
     }
 
     lxw_xml_end_tag(self->file, "numFmts");
diff --git a/src/workbook.c b/src/workbook.c
index 2cfca6dd..f30e3421 100644
--- a/src/workbook.c
+++ b/src/workbook.c
@@ -471,7 +471,7 @@ _prepare_num_formats(lxw_workbook *self)
                 num_format_index = calloc(1, sizeof(uint16_t));
                 *num_format_index = index;
                 format->num_format_index = index;
-                lxw_insert_hash_element(num_formats, num_format,
+                lxw_insert_hash_element(num_formats, format->num_format,
                                         num_format_index,
                                         LXW_FORMAT_FIELD_LEN);
                 index++;
diff --git a/test/functional/src/test_format51.c b/test/functional/src/test_format51.c
new file mode 100644
index 00000000..11ac95ff
--- /dev/null
+++ b/test/functional/src/test_format51.c
@@ -0,0 +1,40 @@
+/*****************************************************************************
+ * Test cases for libxlsxwriter.
+ *
+ * Simple test case to test data writing.
+ *
+ * Copyright 2014-2018, John McNamara, jmcnamara@cpan.org
+ *
+ */
+
+#include "xlsxwriter.h"
+
+
+int main() {
+
+    lxw_workbook  *workbook  = workbook_new("test_format51.xlsx");
+    lxw_worksheet *worksheet = workbook_add_worksheet(workbook, NULL);
+    lxw_format    *format;
+
+    double value = 123.456;
+
+    worksheet_set_column(worksheet, 0, 0, 12, NULL);
+
+    format = workbook_add_format(workbook);
+    format_set_num_format(format, "0.0");
+    worksheet_write_number(worksheet, 0, 0, value, format);
+
+    format = workbook_add_format(workbook);
+    format_set_num_format(format, "0.000");
+    worksheet_write_number(worksheet, 1, 0, value, format);
+
+    format = workbook_add_format(workbook);
+    format_set_num_format(format, "0.0000");
+    worksheet_write_number(worksheet, 2, 0, value, format);
+
+    format = workbook_add_format(workbook);
+    format_set_num_format(format, "0.00000");
+    worksheet_write_number(worksheet, 3, 0, value, format);
+
+    return workbook_close(workbook);
+}
diff --git a/test/functional/src/test_format52.c b/test/functional/src/test_format52.c
new file mode 100644
index 00000000..c0d69155
--- /dev/null
+++ b/test/functional/src/test_format52.c
@@ -0,0 +1,60 @@
+/*****************************************************************************
+ * Test cases for libxlsxwriter.
+ *
+ * Simple test case to test data writing.
+ *
+ * Copyright 2014-2018, John McNamara, jmcnamara@cpan.org
+ *
+ */
+
+#include "xlsxwriter.h"
+
+
+int main() {
+
+    lxw_workbook  *workbook  = workbook_new("test_format52.xlsx");
+    lxw_worksheet *worksheet = workbook_add_worksheet(workbook, NULL);
+    lxw_format    *format;
+
+    double value = 123.456;
+
+    worksheet_set_column(worksheet, 0, 0, 12, NULL);
+
+    format = workbook_add_format(workbook);
+    format_set_num_format(format, "0.0");
+    worksheet_write_number(worksheet, 0, 0, value, format);
+
+    format = workbook_add_format(workbook);
+    format_set_num_format(format, "0.000");
+    worksheet_write_number(worksheet, 1, 0, value, format);
+
+    format = workbook_add_format(workbook);
+    format_set_num_format(format, "0.0000");
+    worksheet_write_number(worksheet, 2, 0, value, format);
+
+    format = workbook_add_format(workbook);
+    format_set_num_format(format, "0.00000");
+    worksheet_write_number(worksheet, 3, 0, value, format);
+
+    format = workbook_add_format(workbook);
+    format_set_num_format(format, "0.0");
+    format_set_bold(format);
+    worksheet_write_number(worksheet, 4, 0, value, format);
+
+    format = workbook_add_format(workbook);
+    format_set_num_format(format, "0.000");
+    format_set_bold(format);
+    worksheet_write_number(worksheet, 5, 0, value, format);
+
+    format = workbook_add_format(workbook);
+    format_set_num_format(format, "0.0000");
+    format_set_bold(format);
+    worksheet_write_number(worksheet, 6, 0, value, format);
+
+    format = workbook_add_format(workbook);
+    format_set_num_format(format, "0.00000");
+    format_set_bold(format);
+    worksheet_write_number(worksheet, 7, 0, value, format);
+
+    return workbook_close(workbook);
+}
diff --git a/test/functional/test_formatting.py b/test/functional/test_formatting.py
index f3324ca7..d86aba99 100644
--- a/test/functional/test_formatting.py
+++ b/test/functional/test_formatting.py
@@ -39,3 +39,9 @@ class TestCompareXLSXFiles(base_test_class.XLSXBaseTest):
 
     def test_format50(self):
         self.run_exe_test('test_format50')
+
+    def test_format51(self):
+        self.run_exe_test('test_format51')
+
+    def test_format52(self):
+        self.run_exe_test('test_format52')
diff --git a/test/functional/xlsx_files/format51.xlsx b/test/functional/xlsx_files/format51.xlsx
new file mode 100644
index 00000000..84181d48
Binary files /dev/null and b/test/functional/xlsx_files/format51.xlsx differ
diff --git a/test/functional/xlsx_files/format52.xlsx b/test/functional/xlsx_files/format52.xlsx
new file mode 100644
index 00000000..54c544f7
Binary files /dev/null and b/test/functional/xlsx_files/format52.xlsx differ