diff --git a/mesonbuild/mesonmain.py b/mesonbuild/mesonmain.py
index dd265c41b74c..ca7a18414ebc 100644
--- a/mesonbuild/mesonmain.py
+++ b/mesonbuild/mesonmain.py
@@ -240,14 +240,14 @@ def validate_original_args(args):
 
     def has_startswith(coll, target):
         for entry in coll:
-            if entry.startswith(target):
+            if entry.startswith(target + '=') or entry == target:
                 return True
         return False
     #ds = [x for x in args if x.startswith('-D')]
     #longs = [x for x in args if x.startswith('--')]
     for optionkey in itertools.chain(mesonbuild.options.BUILTIN_DIR_OPTIONS, mesonbuild.options.BUILTIN_CORE_OPTIONS):
         longarg = mesonbuild.options.argparse_name_to_arg(optionkey.name)
-        shortarg = f'-D{optionkey.name}='
+        shortarg = f'-D{optionkey.name}'
         if has_startswith(args, longarg) and has_startswith(args, shortarg):
             sys.exit(
                 f'Got argument {optionkey.name} as both {shortarg} and {longarg}. Pick one.')
diff --git a/unittests/platformagnostictests.py b/unittests/platformagnostictests.py
index ebd5317a464c..ff2171de6a6b 100644
--- a/unittests/platformagnostictests.py
+++ b/unittests/platformagnostictests.py
@@ -560,3 +560,8 @@ def test_mtest_rebuild_deps(self):
         self.clean()
 
         self._run(self.mtest_command + ['runner-with-exedep'])
+
+    def test_setup_mixed_long_short_options(self) -> None:
+        """Mixing unity and unity_size as long and short options should work."""
+        testdir = self.copy_srcdir(os.path.join(self.common_test_dir, '1 trivial'))
+        self.init(testdir, extra_args=['-Dunity=on', '--unity-size=123'])