diff --git a/distutils/command/build_ext.py b/distutils/command/build_ext.py
index ec45b440..1b4763eb 100644
--- a/distutils/command/build_ext.py
+++ b/distutils/command/build_ext.py
@@ -276,10 +276,10 @@ def finalize_options(self) -> None:  # noqa: C901
         if self.undef:
             self.undef = self.undef.split(',')
 
-        if self.swig_opts is None:
-            self.swig_opts = []
-        else:
+        if isinstance(self.swig_opts, str):
             self.swig_opts = self.swig_opts.split(' ')
+        elif self.swig_opts is None:
+            self.swig_opts = []
 
         # Finally add the user include and library directories if requested
         if self.user:
diff --git a/distutils/tests/test_build_ext.py b/distutils/tests/test_build_ext.py
index dab0507f..c85083e0 100644
--- a/distutils/tests/test_build_ext.py
+++ b/distutils/tests/test_build_ext.py
@@ -320,6 +320,12 @@ def test_finalize_options(self):
         cmd.finalize_options()
         assert cmd.swig_opts == ['1', '2']
 
+        # make sure finalize_options() can be called more than once
+        # without raising an exception
+        cmd = self.build_ext(dist)
+        cmd.finalize_options()
+        cmd.finalize_options()
+
     def test_check_extensions_list(self):
         dist = Distribution()
         cmd = self.build_ext(dist)
diff --git a/newsfragments/385.bugfix.rst b/newsfragments/385.bugfix.rst
new file mode 100644
index 00000000..30e52b16
--- /dev/null
+++ b/newsfragments/385.bugfix.rst
@@ -0,0 +1,2 @@
+Fix a bug where calling ``distutils.build_ext.finalize_options`` more than once
+would raise an exception.
\ No newline at end of file