2 files changed, 87 insertions, 0 deletions
diff --git a/internal/processor/processor.go b/internal/processor/processor.go
index d781a8b..9cbc5a7 100644
--- a/internal/processor/processor.go
+++ b/internal/processor/processor.go
@@ -77,8 +77,12 @@ func Run(cfg *config.Config) (int, error) {
 // claimedByMarkdown scans all .md entries in inputDir and returns a set of
 // image filenames that are referenced within those markdown files.
 // Those images should be embedded in the markdown post, not processed alone.
+// If two different markdown files claim the same image, an error is returned.
 func claimedByMarkdown(entries []os.DirEntry, inputDir string) (map[string]bool, error) {
 	claimed := make(map[string]bool)
+	// owners tracks which markdown file first claimed each image so we can
+	// detect conflicts before processing begins.
+	owners := make(map[string]string)
 
 	for _, entry := range entries {
 		if entry.IsDir() || strings.ToLower(filepath.Ext(entry.Name())) != ".md" {
@@ -92,6 +96,10 @@ func claimedByMarkdown(entries []os.DirEntry, inputDir string) (map[string]bool,
 		}
 
 		for _, imgName := range findLocalImages(string(data), inputDir) {
+			if owner, exists := owners[imgName]; exists && owner != entry.Name() {
+				return nil, fmt.Errorf("image %q claimed by both %q and %q", imgName, owner, entry.Name())
+			}
+			owners[imgName] = entry.Name()
 			claimed[imgName] = true
 		}
 	}
diff --git a/internal/processor/processor_test.go b/internal/processor/processor_test.go
index 3b34675..418fb95 100644
--- a/internal/processor/processor_test.go
+++ b/internal/processor/processor_test.go
@@ -5,6 +5,7 @@ import (
 	"image/png"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -236,3 +237,81 @@ text`
 		t.Fatal(err)
 	}
 }
+
+func TestRun_twoMarkdownsClaimingSameImageFails(t *testing.T) {
+	t.Parallel()
+
+	in := t.TempDir()
+	out := t.TempDir()
+
+	// Shared image in the inbox.
+	pngPath := filepath.Join(in, "pic.png")
+	f, err := os.Create(pngPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := png.Encode(f, image.NewRGBA(image.Rect(0, 0, 2, 2))); err != nil {
+		f.Close()
+		t.Fatal(err)
+	}
+	f.Close()
+
+	// Two markdown files both reference the same image.
+	if err := os.WriteFile(filepath.Join(in, "a.md"), []byte("![a](pic.png)\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(in, "b.md"), []byte("![b](pic.png)\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = Run(&config.Config{InputDir: in, OutputDir: out, BaseURL: "https://x"})
+	if err == nil {
+		t.Fatal("expected error when two markdowns claim the same image")
+	}
+	if !strings.Contains(err.Error(), "pic.png") {
+		t.Fatalf("error should mention the conflicting image, got: %v", err)
+	}
+
+	// Verify that no post directories were created and no source files deleted.
+	entries, _ := os.ReadDir(filepath.Join(out, "posts"))
+	if len(entries) != 0 {
+		t.Fatalf("expected no posts created, got %d", len(entries))
+	}
+	for _, name := range []string{"pic.png", "a.md", "b.md"} {
+		if _, err := os.Stat(filepath.Join(in, name)); err != nil {
+			t.Fatalf("source %s should still exist: %v", name, err)
+		}
+	}
+}
+
+func TestRun_duplicateImageClaimsInSameMarkdownAllowed(t *testing.T) {
+	t.Parallel()
+
+	in := t.TempDir()
+	out := t.TempDir()
+
+	pngPath := filepath.Join(in, "pic.png")
+	f, err := os.Create(pngPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := png.Encode(f, image.NewRGBA(image.Rect(0, 0, 2, 2))); err != nil {
+		f.Close()
+		t.Fatal(err)
+	}
+	f.Close()
+
+	// Same markdown references the same image twice — should not be treated as a conflict.
+	md := "![first](pic.png)\n![second](pic.png)\n"
+	if err := os.WriteFile(filepath.Join(in, "post.md"), []byte(md), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	n, err := Run(&config.Config{InputDir: in, OutputDir: out, BaseURL: "https://x"})
+	if err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+	if n != 1 {
+		t.Fatalf("n=%d; want 1", n)
+	}
+}