diff --git a/src/photools3/metadata_writer.py b/src/photools3/metadata_writer.py
index 06d5fd2..9f8c318 100644
--- a/src/photools3/metadata_writer.py
+++ b/src/photools3/metadata_writer.py
@@ -48,6 +48,11 @@ def _is_prefixed(txt: str) -> bool:
     return txt.startswith(settings.metadata_prefix)
 
 
+def _separate_prefixed_caption(txt: str) -> tuple[str, str | None]:
+    separated = [x.strip() for x in txt.split(settings.metadata_prefix)]
+    return (separated[0], separated[1] if len(separated) > 1 else None)
+
+
 class MetadataWriter:
     """
     Write metadata (captions and keywords) to image files.
@@ -112,14 +117,23 @@ class MetadataWriter:
     def write_caption(
         self, caption: Caption, include_prefix: bool = True, override: bool = True
     ):
-        if include_prefix:
-            caption = _prefixed(caption)
-        existing = self.get_caption()
-        is_existing_prefixed = _is_prefixed(existing)
-        if existing and is_existing_prefixed and not override:
+        existing_no_prefix, existing_prefix = _separate_prefixed_caption(
+            self.get_caption()
+        )
+        if existing_no_prefix:
+            existing_no_prefix = (
+                f"{existing_no_prefix}."
+                if not existing_no_prefix.endswith(".")
+                else existing_no_prefix
+            )
+            existing_no_prefix = f"{existing_no_prefix} "
+
+        if include_prefix and existing_prefix and not override:
             return
-        elif existing and not is_existing_prefixed:
-            caption = f"{existing}. {_prefixed(caption)}"
+        if not include_prefix and existing_no_prefix and not override:
+            return
+        if include_prefix:
+            caption = settings.metadata_prefix.join([existing_no_prefix, f" {caption}"])
         metadata_tags = {key: caption for key in XMP_KEYS_CAPTION}
         self.set_metadata(metadata_tags)
 
diff --git a/test/test_metadata_writer.py b/test/test_metadata_writer.py
index 4c7405b..7597d59 100644
--- a/test/test_metadata_writer.py
+++ b/test/test_metadata_writer.py
@@ -21,6 +21,16 @@ def make_dng_file(tmp_path):
     return _make
 
 
+@pytest.fixture
+def metadata_writer_on_existing(make_dng_file):
+    metadata_writer = MetadataWriter(make_dng_file(PATHS_DNG_EXAMPLES[0]))
+    metadata_writer.write_caption(
+        "Let's pretend this is the caption of the original image",
+        include_prefix=False,
+    )
+    return metadata_writer
+
+
 @pytest.mark.parametrize("pic", PATHS_DNG_EXAMPLES)
 def test_read_metadata(make_dng_file, pic):
     """primitive test for `MetadataWriter`"""
@@ -49,22 +59,47 @@ def test_is_prefixed():
 def test_read_metadata_sidecar_path(make_dng_file, pic):
     file_path = make_dng_file(pic)
     metadata_writer = MetadataWriter(file_path)
-    print(metadata_writer.sidecar_path)
     assert isinstance(metadata_writer.sidecar_path, Path)
     assert str(metadata_writer.sidecar_path).lower().endswith(".dng.xmp")
 
+    # file does not exits yet, only the name
+    assert not metadata_writer.sidecar_path.exists()
+
 
 @pytest.mark.parametrize("pic", PATHS_DNG_EXAMPLES)
 def test_set_caption(make_dng_file, pic):
     metadata_writer = MetadataWriter(make_dng_file(pic))
+    assert not metadata_writer.sidecar_path.exists()  # sidecar does not exist yet ...
     metadata_writer.write_caption("This is a test")
+    assert metadata_writer.sidecar_path.exists()  # ... but now!
     result = metadata_writer.get_caption()
     assert result
+    assert result == "photools: This is a test"
 
 
-def test_set_caption_on_existing():
-    # TODO
-    ...
+def test_set_caption_on_existing(metadata_writer_on_existing):
+    assert (
+        metadata_writer_on_existing.get_caption()
+        == "Let's pretend this is the caption of the original image"
+    )
+
+    metadata_writer_on_existing.write_caption("Another caption")
+    assert (
+        metadata_writer_on_existing.get_caption()
+        == "Let's pretend this is the caption of the original image. photools: Another caption"
+    )
+
+    metadata_writer_on_existing.write_caption("Updated caption")
+    assert (
+        metadata_writer_on_existing.get_caption()
+        == "Let's pretend this is the caption of the original image. photools: Updated caption"
+    )
+
+    metadata_writer_on_existing.write_caption("Don't do this!", override=False)
+    assert (
+        metadata_writer_on_existing.get_caption()
+        == "Let's pretend this is the caption of the original image. photools: Updated caption"
+    )
 
 
 @pytest.mark.parametrize("pic", PATHS_DNG_EXAMPLES)