diff --git a/README.md b/README.md
index b673799..8dca315 100644
--- a/README.md
+++ b/README.md
@@ -11,7 +11,7 @@ go run . <command>
 ## Commands
 
 - `sigil apply` - apply symlinks from `~/.dotfiles`
-- `sigil add <path>` - add an existing file or folder
+- `sigil add <path>` - add an existing file or folder (merges into existing package for single files)
 
 ## Repo layout
 
diff --git a/main.go b/main.go
index 4b86754..cc5218a 100644
--- a/main.go
+++ b/main.go
@@ -247,8 +247,9 @@ func addCmd(args []string) error {
 	}
 
 	pkgDir := filepath.Join(repo, pkgName)
+	pkgExists := false
 	if _, err := os.Stat(pkgDir); err == nil {
-		return fmt.Errorf("package %q already exists", pkgName)
+		pkgExists = true
 	} else if !errors.Is(err, os.ErrNotExist) {
 		return err
 	}
@@ -259,11 +260,15 @@ func addCmd(args []string) error {
 	}
 
 	configPath := filepath.Join(pkgDir, configFileName)
-	if err := writeConfig(configPath, targetRoot); err != nil {
-		return err
+	if !pkgExists {
+		if err := writeConfig(configPath, targetRoot); err != nil {
+			return err
+		}
 	}
-
 	if info.IsDir() {
+		if pkgExists {
+			return errors.New("cannot merge a directory into an existing package yet")
+		}
 		if err := moveDirContents(absPath, filesDir); err != nil {
 			return err
 		}
@@ -280,6 +285,11 @@ func addCmd(args []string) error {
 		if err := os.MkdirAll(filepath.Dir(destPath), 0o755); err != nil {
 			return err
 		}
+		if _, err := os.Stat(destPath); err == nil {
+			return fmt.Errorf("destination already exists: %s", destPath)
+		} else if !errors.Is(err, os.ErrNotExist) {
+			return err
+		}
 		if err := os.Rename(absPath, destPath); err != nil {
 			return err
 		}