diff --git a/src/deb/local.rs b/src/deb/local.rs
index 1c05552..9743b3e 100644
--- a/src/deb/local.rs
+++ b/src/deb/local.rs
@@ -512,6 +512,23 @@ fn apply_quilt_patches(
         return Ok(());
     }
 
+    // A `single-debian-patch` tree is already patched by construction:
+    // `dpkg-source -b` folds the working-tree delta into
+    // 'debian/patches/debian-changes' and registers it in the series
+    // WITHOUT applying it — the content stays ambient in the working tree
+    // (e.g. the vendored rust '.cargo/config.toml'). Raw `quilt push -a`
+    // would then refuse the patch ('file already exists'); only dpkg's own
+    // patch(1)-based application tolerates that. Since `dpkg-source -b`
+    // regenerates the patch from the working tree at source-build time,
+    // applying patches is wrong in principle here: skip the step.
+    if uses_single_debian_patch(&ctx, Path::new(package_dir)) {
+        log::info!(
+            "Tree uses single-debian-patch: the working tree already \
+             carries the patch content, skipping quilt patch application"
+        );
+        return Ok(());
+    }
+
     // Make sure quilt is available in the build context
     log::debug!("Installing quilt for patch application...");
     let status = cap(
@@ -552,6 +569,39 @@ fn apply_quilt_patches(
     Ok(())
 }
 
+/// Whether the source options of the package tree at `package_dir` (inside
+/// the build context) declare the `single-debian-patch` mode: either
+/// `debian/source/local-options` or `debian/source/options` contains a
+/// line whose trimmed content — after stripping a leading `--` long-option
+/// dash — is exactly `single-debian-patch` (both spellings exist in the
+/// wild). A line merely containing the token as a substring (e.g.
+/// `--single-debian-patch-foo`) does not count.
+fn uses_single_debian_patch(ctx: &Context, package_dir: &Path) -> bool {
+    let local_options = ctx
+        .read_file(&package_dir.join("debian/source/local-options"))
+        .ok();
+    let options = ctx
+        .read_file(&package_dir.join("debian/source/options"))
+        .ok();
+    options_declare_single_debian_patch(local_options.as_deref(), options.as_deref())
+}
+
+/// Pure decision core of [`uses_single_debian_patch`]: do the (optional)
+/// contents of `debian/source/local-options` / `debian/source/options`
+/// declare `single-debian-patch`?
+fn options_declare_single_debian_patch(local_options: Option<&str>, options: Option<&str>) -> bool {
+    let declares = |content: Option<&str>| {
+        content.is_some_and(|content| {
+            content.lines().any(|line| {
+                let line = line.trim();
+                let line = line.strip_prefix("--").unwrap_or(line);
+                line == "single-debian-patch"
+            })
+        })
+    };
+    declares(local_options) || declares(options)
+}
+
 /// Pin a 'NotAutomatic' pocket (e.g. '-proposed') so apt takes it into
 /// account during dependency resolution.
 ///
@@ -685,3 +735,116 @@ fn dose3_explain_dependencies(
     cmd.status()?;
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::context::ContextConfig;
+
+    #[test]
+    fn detector_matches_local_options_options_and_dashed_spellings() {
+        // The pkh scaffold spelling: bare token in local-options.
+        assert!(options_declare_single_debian_patch(
+            Some("single-debian-patch\n"),
+            None
+        ));
+        // The other common spelling in either file: `--`-prefixed.
+        assert!(options_declare_single_debian_patch(
+            Some("--single-debian-patch\n"),
+            None
+        ));
+        assert!(options_declare_single_debian_patch(
+            None,
+            Some("--single-debian-patch\n")
+        ));
+        // Trailing whitespace and blank lines around the token.
+        assert!(options_declare_single_debian_patch(
+            Some("\n  single-debian-patch  \n"),
+            None
+        ));
+        // Declared in options while local-options carries other options.
+        assert!(options_declare_single_debian_patch(
+            Some("--extend-diff-ignore='^vendor/'\n"),
+            Some("single-debian-patch\n")
+        ));
+    }
+
+    #[test]
+    fn detector_rejects_substrings_empty_and_absent_files() {
+        // The token must not match as a substring of another option.
+        assert!(!options_declare_single_debian_patch(
+            Some("--single-debian-patch-ignore=^foo\n"),
+            None
+        ));
+        assert!(!options_declare_single_debian_patch(
+            Some("--no-single-debian-patch\n"),
+            Some("single-debian-patching\n")
+        ));
+        // Absent or empty files: not single-debian-patch.
+        assert!(!options_declare_single_debian_patch(None, None));
+        assert!(!options_declare_single_debian_patch(Some(""), Some("")));
+        // Unrelated content.
+        assert!(!options_declare_single_debian_patch(
+            Some("--extend-diff-ignore='^vendor/'\n"),
+            Some("tar-ignore = .git\n")
+        ));
+    }
+
+    /// A tree with `single-debian-patch` in its local-options is detected
+    /// through the real build-context file access; a tree without the
+    /// declaration is not.
+    #[test]
+    fn detector_on_a_real_tree_through_the_context() {
+        let ctx = Context::new(ContextConfig::Local).unwrap();
+
+        let dir = tempfile::tempdir().unwrap();
+        let tree = dir.path().join("pkg");
+        std::fs::create_dir_all(tree.join("debian/source")).unwrap();
+        std::fs::write(
+            tree.join("debian/source/local-options"),
+            "single-debian-patch\n",
+        )
+        .unwrap();
+        assert!(uses_single_debian_patch(&ctx, &tree));
+
+        // No source options at all.
+        let dir = tempfile::tempdir().unwrap();
+        let tree = dir.path().join("pkg");
+        std::fs::create_dir_all(tree.join("debian/source")).unwrap();
+        assert!(!uses_single_debian_patch(&ctx, &tree));
+    }
+
+    /// Direct call of the patch-application phase on a tree simulating the
+    /// `dpkg-source -b` state of a single-debian-patch scaffold:
+    /// 'debian-changes' is registered in the series while its content is
+    /// already ambient in the working tree (raw `quilt push -a` would
+    /// refuse it with 'file already exists'). The phase must skip the
+    /// application entirely and succeed.
+    #[test]
+    fn apply_phase_skips_single_debian_patch_trees() {
+        let ctx = Arc::new(Context::new(ContextConfig::Local).unwrap());
+
+        // Dashed spelling through debian/source/options this time.
+        let dir = tempfile::tempdir().unwrap();
+        let tree = dir.path().join("pkg");
+        std::fs::create_dir_all(tree.join("debian/patches")).unwrap();
+        std::fs::create_dir_all(tree.join("debian/source")).unwrap();
+        // The touched file already carries the patched content: applying
+        // the patch would fail ('README' already exists / differs only in
+        // the patch's imagination).
+        std::fs::write(tree.join("README"), "patched\n").unwrap();
+        std::fs::write(
+            tree.join("debian/patches/debian-changes"),
+            "--- a/README\n+++ b/README\n@@ -1 +1 @@\n-orig\n+patched\n",
+        )
+        .unwrap();
+        std::fs::write(tree.join("debian/patches/series"), "debian-changes\n").unwrap();
+        std::fs::write(
+            tree.join("debian/source/options"),
+            "--single-debian-patch\n",
+        )
+        .unwrap();
+
+        apply_quilt_patches(tree.to_str().unwrap(), &HashMap::new(), ctx, &None, &None).unwrap();
+    }
+}