diff --git a/lib/puppet/functions/mysql/normalise_and_deepmerge.rb b/lib/puppet/functions/mysql/normalise_and_deepmerge.rb
new file mode 100644
index 000000000..70c03f599
--- /dev/null
+++ b/lib/puppet/functions/mysql/normalise_and_deepmerge.rb
@@ -0,0 +1,67 @@
+# @summary Recursively merges two or more hashes together, normalises keys with differing use of dashesh and underscores,
+#   then returns the resulting hash.
+#
+# @example
+#   $hash1 = {'one' => 1, 'two' => 2, 'three' => { 'four' => 4 } }
+#   $hash2 = {'two' => 'dos', 'three' => { 'five' => 5 } }
+#   $merged_hash = mysql::normalise_and_deepmerge($hash1, $hash2)
+#   # The resulting hash is equivalent to:
+#   # $merged_hash = { 'one' => 1, 'two' => 'dos', 'three' => { 'four' => 4, 'five' => 5 } }
+#
+# - When there is a duplicate key that is a hash, they are recursively merged.
+# - When there is a duplicate key that is not a hash, the key in the rightmost hash will "win."
+# - When there are conficting uses of dashes and underscores in two keys (which mysql would otherwise equate), the rightmost style will win.
+#
+Puppet::Functions.create_function(:'mysql::normalise_and_deepmerge') do
+  def normalise_and_deepmerge(*args)
+    if args.length < 2
+      raise Puppet::ParseError, _('mysql::normalise_and_deepmerge(): wrong number of arguments (%{args_length}; must be at least 2)') % { args_length: args.length }
+    end
+
+    result = {}
+    args.each do |arg|
+      next if arg.is_a?(String) && arg.empty? # empty string is synonym for puppet's undef
+      # If the argument was not a hash, skip it.
+      unless arg.is_a?(Hash)
+        raise Puppet::ParseError, _('mysql::normalise_and_deepmerge: unexpected argument type %{arg_class}, only expects hash arguments.') % { args_class: args.class }
+      end
+
+      # We need to make a copy of the hash since it is frozen by puppet
+      current = deep_copy(arg)
+
+      # Now we have to traverse our hash assigning our non-hash values
+      # to the matching keys in our result while following our hash values
+      # and repeating the process.
+      overlay(result, current)
+    end
+    result
+  end
+
+  def normalized?(hash, key)
+    return true if hash.key?(key)
+    return false unless key =~ %r{-|_}
+    other_key = key.include?('-') ? key.tr('-', '_') : key.tr('_', '-')
+    return false unless hash.key?(other_key)
+    hash[key] = hash.delete(other_key)
+    true
+  end
+
+  def overlay(hash1, hash2)
+    hash2.each do |key, value|
+      if normalized?(hash1, key) && value.is_a?(Hash) && hash1[key].is_a?(Hash)
+        overlay(hash1[key], value)
+      else
+        hash1[key] = value
+      end
+    end
+  end
+
+  def deep_copy(inputhash)
+    return inputhash unless inputhash.is_a? Hash
+    hash = {}
+    inputhash.each do |k, v|
+      hash.store(k, deep_copy(v))
+    end
+    hash
+  end
+end
diff --git a/manifests/backup/mysqlbackup.pp b/manifests/backup/mysqlbackup.pp
index 24fc99ad7..0da68bf19 100644
--- a/manifests/backup/mysqlbackup.pp
+++ b/manifests/backup/mysqlbackup.pp
@@ -93,7 +93,7 @@
       'password'               => $backuppassword,
     }
   }
-  $options = $default_options.deep_merge($mysql::server::override_options)
+  $options = mysql::normalise_and_deepmerge($default_options, $mysql::server::override_options)
 
   file { 'mysqlbackup-config-file':
     path    => '/etc/mysql/conf.d/meb.cnf',
diff --git a/manifests/server.pp b/manifests/server.pp
index 1f3e5d3be..f70ca4146 100644
--- a/manifests/server.pp
+++ b/manifests/server.pp
@@ -116,7 +116,7 @@
   }
 
   # Create a merged together set of options.  Rightmost hashes win over left.
-  $options = $mysql::params::default_options.deep_merge($override_options)
+  $options = mysql::normalise_and_deepmerge($mysql::params::default_options, $override_options)
 
   Class['mysql::server::root_password'] -> Mysql::Db <| |>
 
diff --git a/spec/functions/mysql_normalise_and_deepmerge_spec.rb b/spec/functions/mysql_normalise_and_deepmerge_spec.rb
new file mode 100644
index 000000000..d1c94f8ff
--- /dev/null
+++ b/spec/functions/mysql_normalise_and_deepmerge_spec.rb
@@ -0,0 +1,92 @@
+require 'spec_helper'
+
+describe 'mysql::normalise_and_deepmerge' do
+  it 'exists' do
+    is_expected.not_to eq(nil)
+  end
+
+  it 'throws error with no arguments' do
+    is_expected.to run.with_params.and_raise_error(Puppet::ParseError)
+  end
+
+  it 'throws error with only one argument' do
+    is_expected.to run.with_params('one' => 1).and_raise_error(Puppet::ParseError)
+  end
+
+  it 'accepts empty strings as puppet undef' do
+    is_expected.to run.with_params({}, '')
+  end
+
+  # rubocop:disable RSpec/NamedSubject
+  index_values = ['one', 'two', 'three']
+  expected_values_one = ['1', '2', '2']
+  it 'merge two hashes' do
+    new_hash = subject.execute({ 'one' => '1', 'two' => '1' }, 'two' => '2', 'three' => '2')
+    index_values.each_with_index do |index, expected|
+      expect(new_hash[index]).to eq(expected_values_one[expected])
+    end
+  end
+
+  it 'merges multiple hashes' do
+    hash = subject.execute({ 'one' => 1 }, { 'one' => '2' }, 'one' => '3')
+    expect(hash['one']).to eq('3')
+  end
+
+  it 'accepts empty hashes' do
+    is_expected.to run.with_params({}, {}, {}).and_return({})
+  end
+
+  expected_values_two = [1, 2, 'four' => 4]
+  it 'merges subhashes' do
+    hash = subject.execute({ 'one' => 1 }, 'two' => 2, 'three' => { 'four' => 4 })
+    index_values.each_with_index do |index, expected|
+      expect(hash[index]).to eq(expected_values_two[expected])
+    end
+  end
+
+  it 'appends to subhashes' do
+    hash = subject.execute({ 'one' => { 'two' => 2 } }, 'one' => { 'three' => 3 })
+    expect(hash['one']).to eq('two' => 2, 'three' => 3)
+  end
+
+  expected_values_three = [1, 'dos', { 'four' => 4, 'five' => 5 }]
+  it 'appends to subhashes 2' do
+    hash = subject.execute({ 'one' => 1, 'two' => 2, 'three' => { 'four' => 4 } }, 'two' => 'dos', 'three' => { 'five' => 5 })
+    index_values.each_with_index do |index, expected|
+      expect(hash[index]).to eq(expected_values_three[expected])
+    end
+  end
+
+  index_values_two = ['key1', 'key2']
+  expected_values_four = [{ 'a' => 1, 'b' => 99 }, 'c' => 3]
+  it 'appends to subhashes 3' do
+    hash = subject.execute({ 'key1' => { 'a' => 1, 'b' => 2 }, 'key2' => { 'c' => 3 } }, 'key1' => { 'b' => 99 })
+    index_values_two.each_with_index do |index, expected|
+      expect(hash[index]).to eq(expected_values_four[expected])
+    end
+  end
+
+  it 'equates keys mod dash and underscore #value' do
+    hash = subject.execute({ 'a-b-c' => 1 }, 'a_b_c' => 10)
+    expect(hash['a_b_c']).to eq(10)
+  end
+  it 'equates keys mod dash and underscore #not' do
+    hash = subject.execute({ 'a-b-c' => 1 }, 'a_b_c' => 10)
+    expect(hash).not_to have_key('a-b-c')
+  end
+
+  index_values_three = ['a_b_c', 'b-c-d']
+  expected_values_five = [10, { 'e-f-g' => 3, 'c_d_e' => 12 }]
+  index_values_error = ['a-b-c', 'b_c_d']
+  index_values_three.each_with_index do |index, expected|
+    it 'keeps style of the last when keys are equal mod dash and underscore #value' do
+      hash = subject.execute({ 'a-b-c' => 1, 'b_c_d' => { 'c-d-e' => 2, 'e-f-g' => 3 } }, 'a_b_c' => 10, 'b-c-d' => { 'c_d_e' => 12 })
+      expect(hash[index]).to eq(expected_values_five[expected])
+    end
+    it 'keeps style of the last when keys are equal mod dash and underscore #not' do
+      hash = subject.execute({ 'a-b-c' => 1, 'b_c_d' => { 'c-d-e' => 2, 'e-f-g' => 3 } }, 'a_b_c' => 10, 'b-c-d' => { 'c_d_e' => 12 })
+      expect(hash).not_to have_key(index_values_error[expected])
+    end
+  end
+  # rubocop:enable RSpec/NamedSubject
+end