diff --git a/.changeset/happy-parts-search.md b/.changeset/happy-parts-search.md
new file mode 100644
index 000000000000..65fb87b4fa5d
--- /dev/null
+++ b/.changeset/happy-parts-search.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: strip field prefix before erroring on duplicates
diff --git a/packages/kit/src/runtime/form-utils.js b/packages/kit/src/runtime/form-utils.js
index 862609c94555..3115f146deba 100644
--- a/packages/kit/src/runtime/form-utils.js
+++ b/packages/kit/src/runtime/form-utils.js
@@ -42,10 +42,6 @@ export function convert_formdata(data) {
 
 		if (is_array) key = key.slice(0, -2);
 
-		if (values.length > 1 && !is_array) {
-			throw new Error(`Form cannot contain duplicated keys — "${key}" has ${values.length} values`);
-		}
-
 		// an empty `<input type="file">` will submit a non-existent file, bizarrely
 		values = values.filter(
 			(entry) => typeof entry === 'string' || entry.name !== '' || entry.size > 0
@@ -60,6 +56,10 @@ export function convert_formdata(data) {
 			values = values.map((v) => v === 'on');
 		}
 
+		if (values.length > 1 && !is_array) {
+			throw new Error(`Form cannot contain duplicated keys — "${key}" has ${values.length} values`);
+		}
+
 		set_nested_value(result, key, is_array ? values : values[0]);
 	}