diff --git a/bibcop.pl b/bibcop.pl
index 05af914..574f090 100755
--- a/bibcop.pl
+++ b/bibcop.pl
@@ -969,9 +969,15 @@ sub fail {
   my @entries = entries($bib);
   if (exists $args{'--fix'}) {
     my $fixed = '';
+    my %seen;
     for my $i (0..(@entries+0 - 1)) {
       my %entry = %{ $entries[$i] };
+      my $name = $entry{':name'};
+      if (exists $seen{$name}) {
+        next;
+      }
       $fixed = $fixed . entry_fix(%entry);
+      $seen{$name} = 1;
     }
     if (exists $args{'-i'} or exists $args{'--in-place'}) {
       open(my $out, '>', $file) or error('Cannot open file for writing: ' . $file);
diff --git a/perl-tests/cli_fixing.pl b/perl-tests/cli_fixing.pl
new file mode 100644
index 0000000..8c28ba6
--- /dev/null
+++ b/perl-tests/cli_fixing.pl
@@ -0,0 +1,65 @@
+#!/usr/bin/perl
+# (The MIT License)
+#
+# Copyright (c) 2022-2024 Yegor Bugayenko
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the 'Software'), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in all
+# copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
+package bibcop;
+
+use strict;
+use warnings;
+use File::Temp qw/ tempfile /;
+
+assert_fix_compare('duplicates');
+assert_fix_compare('no-fixes');
+
+sub read_file {
+  my ($path) = @_;
+  open(my $h, '<', $path) or error('Cannot open file: ' . $path);
+  my $content; { local $/; $content = <$h>; }
+  $content =~ s/^\s+|\s+$//g;
+  return $content;
+}
+
+sub assert_fix_compare {
+  my ($dir) = @_;
+  my $before = "test-files/fixes/$dir/before.bib";
+  my $expected = "test-files/fixes/$dir/expected.bib";
+  my ($a, $after) = tempfile();
+  my $cmd = "perl ./bibcop.pl --fix $before > $after";
+  my $stdout = `$cmd`;
+  my $exit = $? >> 8;
+  if ($exit != 0) {
+    print "---\n$stdout\n";
+    print "Exit code is $exit (not zero) for '$cmd'\n";
+    exit 1;
+  }
+  my $expected_bib = read_file($expected);
+  my $after_bib = read_file($after);
+  if (not $expected_bib eq $after_bib) {
+    print "$after_bib\n";
+    print "Doesn't match:\n";
+    print "$expected_bib\n";
+    exit 1;
+  }
+}
+
+
+1;
diff --git a/test-files/fixes/duplicates/before.bib b/test-files/fixes/duplicates/before.bib
new file mode 100644
index 0000000..8f15d67
--- /dev/null
+++ b/test-files/fixes/duplicates/before.bib
@@ -0,0 +1,14 @@
+@book{knuth1974,
+  author = {Knuth, Donald E. and Duane, Bibby},
+  doi = {10.5555/1102013},
+  publisher = {Addison-Wesley Professional},
+  title = {{The TeX Book}},
+  year = {1984},
+}
+
+@book{knuth1974,
+  author = {Donald E. Knuth},
+  publisher = {Addison-Wesley},
+  title = {The TeX Book},
+  year = {1984},
+}
\ No newline at end of file
diff --git a/test-files/fixes/duplicates/expected.bib b/test-files/fixes/duplicates/expected.bib
new file mode 100644
index 0000000..68ef31f
--- /dev/null
+++ b/test-files/fixes/duplicates/expected.bib
@@ -0,0 +1,7 @@
+@book{knuth1974,
+  author = {Knuth, Donald E. and Duane, Bibby},
+  doi = {10.5555/1102013},
+  publisher = {Addison-Wesley Professional},
+  title = {{The TeX Book}},
+  year = {1984},
+}
\ No newline at end of file
diff --git a/test-files/fixes/no-fixes/before.bib b/test-files/fixes/no-fixes/before.bib
new file mode 100644
index 0000000..68ef31f
--- /dev/null
+++ b/test-files/fixes/no-fixes/before.bib
@@ -0,0 +1,7 @@
+@book{knuth1974,
+  author = {Knuth, Donald E. and Duane, Bibby},
+  doi = {10.5555/1102013},
+  publisher = {Addison-Wesley Professional},
+  title = {{The TeX Book}},
+  year = {1984},
+}
\ No newline at end of file
diff --git a/test-files/fixes/no-fixes/expected.bib b/test-files/fixes/no-fixes/expected.bib
new file mode 100644
index 0000000..68ef31f
--- /dev/null
+++ b/test-files/fixes/no-fixes/expected.bib
@@ -0,0 +1,7 @@
+@book{knuth1974,
+  author = {Knuth, Donald E. and Duane, Bibby},
+  doi = {10.5555/1102013},
+  publisher = {Addison-Wesley Professional},
+  title = {{The TeX Book}},
+  year = {1984},
+}
\ No newline at end of file
diff --git a/test-files/test.bib b/test-files/test.bib
index ee67bcd..1d45299 100644
--- a/test-files/test.bib
+++ b/test-files/test.bib
@@ -1,5 +1,5 @@
-% This is a test file, which is perfectly formatted.
-% Bibcop must not complain about it.
+% This is a test file, which is NOT perfectly formatted.
+% Bibcop must complain about certain parts of it (see perl-tests/cli.pl).
 
 @book{knuth1974,
   author = {Knuth, Donald E. and Duane, Bibby},
diff --git a/tests.pl b/tests.pl
index add7d6c..c0e6900 100755
--- a/tests.pl
+++ b/tests.pl
@@ -63,6 +63,7 @@ sub show {
 require './perl-tests/functions.pl';
 require './perl-tests/checks.pl';
 require './perl-tests/cli.pl';
+require './perl-tests/cli_fixing.pl';
 
 print "\033[0;32mGREAT!\033[0m All tests are green.\n";