diff --git a/examples/issue-0056-diamond-param-impl.sx b/examples/issue-0056-diamond-param-impl.sx
new file mode 100644
index 0000000..ddcf0c6
--- /dev/null
+++ b/examples/issue-0056-diamond-param-impl.sx
@@ -0,0 +1,19 @@
+// Regression (issue 0056): a parameterised-protocol (`Into`) impl living in a
+// module reached through more than one import path must register exactly once.
+//
+//   main ─┬─ issue-0056/mid_a ─┐
+//         └─ issue-0056/mid_b ─┴─ issue-0056/common  (holds `impl Into(Wrapped) for s64`)
+//
+// Impl blocks are anonymous (no `declName`), so before the fix the diamond
+// dedup in imports.zig appended the cached node once per path and the second
+// registration tripped: `duplicate impl 'Into' for source 's64'`. Now the flat
+// decl list also dedups by node identity, so this builds and prints 7.
+#import "modules/std.sx";
+#import "issue-0056/mid_a.sx";
+#import "issue-0056/mid_b.sx";
+
+main :: () -> s32 {
+    w : Wrapped = xx 7;
+    print("{}\n", w.v);
+    0;
+}
diff --git a/examples/issue-0056/common.sx b/examples/issue-0056/common.sx
new file mode 100644
index 0000000..6c18d61
--- /dev/null
+++ b/examples/issue-0056/common.sx
@@ -0,0 +1,12 @@
+// A parameterised-protocol (`Into`) impl living in a module reachable through
+// more than one import path. Each path must NOT re-register the impl.
+// Impl blocks are anonymous (`declName() == null`), so the diamond-import
+// dedup in imports.zig (`mergeFlat`) skips them and appends the node once per
+// path — `registerParamImpl` then trips its same-file duplicate check.
+Wrapped :: struct { v: s64; }
+
+impl Into(Wrapped) for s64 {
+    convert :: (self: s64) -> Wrapped {
+        return .{ v = self };
+    }
+}
diff --git a/examples/issue-0056/mid_a.sx b/examples/issue-0056/mid_a.sx
new file mode 100644
index 0000000..a7a5ea8
--- /dev/null
+++ b/examples/issue-0056/mid_a.sx
@@ -0,0 +1,3 @@
+#import "common.sx";
+
+mid_a_marker :: () -> s64 { 1; }
diff --git a/examples/issue-0056/mid_b.sx b/examples/issue-0056/mid_b.sx
new file mode 100644
index 0000000..4708cfe
--- /dev/null
+++ b/examples/issue-0056/mid_b.sx
@@ -0,0 +1,3 @@
+#import "common.sx";
+
+mid_b_marker :: () -> s64 { 2; }
diff --git a/issues/0056-param-impl-not-deduped-across-diamond-import.md b/issues/0056-param-impl-not-deduped-across-diamond-import.md
new file mode 100644
index 0000000..9457cf4
--- /dev/null
+++ b/issues/0056-param-impl-not-deduped-across-diamond-import.md
@@ -0,0 +1,79 @@
+# 0056 — parameterised-protocol impl not deduped across a diamond import
+
+**FIXED** (`examples/issue-0056-diamond-param-impl.sx`, helpers in
+`examples/issue-0056/`). The flat decl list in
+[src/imports.zig](../src/imports.zig) now dedups by **node identity** as well
+as by name. `ResolvedModule.mergeFlat` and the directory-import merge loop each
+carry a `seen_nodes: std.AutoHashMap(*Node, void)` alongside the existing
+`seen_in_list` name set, and skip a decl whose pointer was already appended.
+
+## Symptom
+
+A module containing a parameterised-protocol impl (`impl Into(T) for S`) could
+not be imported through more than one path. Under a diamond —
+
+```
+main ─┬─ mid_a ─┐
+      └─ mid_b ─┴─ common   (holds `impl Into(Wrapped) for s64`)
+```
+
+— compilation failed with:
+
+```
+error: duplicate impl 'Into' for source 's64' in .../common.sx
+```
+
+This bit the moment `modules/std/objc.sx` (imported by `main.sx`,
+`platform/uikit.sx`, and `gpu/metal.sx` — a diamond) gained an
+`impl Into(*NSString) for string`.
+
+## Root cause
+
+`mergeFlat`/`addOwnDecl` dedup the global flat decl list by
+`decl.data.declName()`. Named decls (structs, fns, foreign classes) dedup
+fine across diamonds. But `impl_block` is anonymous — `declName()` returns
+`null` (see [src/ast.zig](../src/ast.zig) `Data.declName`) — so the dedup
+guard was skipped and the **same cached** impl node (modules are cached in
+`ModuleCache`, so both paths share one node pointer) was appended once per
+path. `registerParamImpl` in [src/ir/lower.zig](../src/ir/lower.zig) then saw
+two entries with the same `defining_module` and raised the same-file
+"duplicate impl" diagnostic.
+
+The pre-existing `impl Into(Block) for Closure(...)` in
+`modules/std/objc_block.sx` never tripped this because that module is not
+imported through any diamond.
+
+## Reproduction
+
+`examples/issue-0056/common.sx`:
+
+```sx
+Wrapped :: struct { v: s64; }
+
+impl Into(Wrapped) for s64 {
+    convert :: (self: s64) -> Wrapped {
+        return .{ v = self };
+    }
+}
+```
+
+`mid_a.sx` / `mid_b.sx` each `#import "common.sx";`. The diamond main:
+
+```sx
+#import "modules/std.sx";
+#import "issue-0056/mid_a.sx";
+#import "issue-0056/mid_b.sx";
+
+main :: () -> s32 {
+    w : Wrapped = xx 7;   // pre-fix: duplicate impl 'Into' for source 's64'
+    print("{}\n", w.v);   // post-fix: prints 7
+    0;
+}
+```
+
+## Fix
+
+Dedup the flat decl list by node identity in addition to name — a physical
+AST node must be lowered once regardless of how many import paths reach it.
+Named-decl first-wins behaviour is unchanged. Regression test:
+`examples/issue-0056-diamond-param-impl.sx` (in `tests/run_examples.sh`).
diff --git a/src/imports.zig b/src/imports.zig
index 4d158c0..5ce9de5 100644
--- a/src/imports.zig
+++ b/src/imports.zig
@@ -315,20 +315,29 @@ pub const ResolvedModule = struct {
     /// `self.scope` — visibility joins per-file scopes at lookup time via
     /// `import_graph`. We only need to append `other.decls` (the full
     /// transitive list) to the global `list` so the lowering pass can
-    /// still resolve transitively-imported callees. Deduped by name.
+    /// still resolve transitively-imported callees.
+    ///
+    /// Deduped two ways: named decls by name (first-wins on cross-module
+    /// collisions), and EVERY decl by node identity. The latter matters for
+    /// anonymous decls — `impl` blocks have no `declName`, so under a diamond
+    /// import the same cached node would otherwise be appended once per path
+    /// and registered twice (e.g. `duplicate impl 'Into'`).
     pub fn mergeFlat(
         self: *ResolvedModule,
         allocator: std.mem.Allocator,
         list: *std.ArrayList(*Node),
         seen_list: *std.StringHashMap(void),
+        seen_nodes: *std.AutoHashMap(*Node, void),
         other: ResolvedModule,
     ) !void {
         _ = self;
         for (other.decls) |decl| {
+            if (seen_nodes.contains(decl)) continue;
             if (decl.data.declName()) |name| {
                 if (seen_list.contains(name)) continue;
                 try seen_list.put(name, {});
             }
+            try seen_nodes.put(decl, {});
             try list.append(allocator, decl);
         }
     }
@@ -420,6 +429,9 @@ pub fn resolveImports(
     // by `mergeFlat` to dedupe across diamond imports now that `mod.scope`
     // is non-transitive and can no longer serve as the dedup key.
     var seen_in_list = std.StringHashMap(void).init(allocator);
+    // Node-identity set for the same purpose, covering anonymous decls
+    // (impl blocks) that carry no name to dedupe on.
+    var seen_nodes = std.AutoHashMap(*Node, void).init(allocator);
 
     for (flat_decls) |decl| {
         if (decl.data == .c_import_decl) {
@@ -559,7 +571,7 @@ pub fn resolveImports(
         if (imp.name) |ns_name| {
             try mod.addNamespace(allocator, &decl_list, &own_decl_list, &seen_in_list, ns_name, imported_mod, decl.span);
         } else {
-            try mod.mergeFlat(allocator, &decl_list, &seen_in_list, imported_mod);
+            try mod.mergeFlat(allocator, &decl_list, &seen_in_list, &seen_nodes, imported_mod);
         }
     }
 
@@ -622,6 +634,7 @@ fn resolveDirectoryImport(
     var decl_list = std.ArrayList(*Node).empty;
     var own_decl_list = std.ArrayList(*Node).empty;
     var seen_in_list = std.StringHashMap(void).init(allocator);
+    var seen_nodes = std.AutoHashMap(*Node, void).init(allocator);
 
     for (file_names.items) |file_name| {
         const file_path = try std.fmt.allocPrint(allocator, "{s}/{s}", .{ dir_path, file_name });
@@ -668,10 +681,12 @@ fn resolveDirectoryImport(
         // register `Event` (a tagged_union) before `handle_event(e: *Event)`
         // triggers the placeholder-struct fallback in `resolveTypeName`.
         for (file_mod.decls) |decl| {
+            if (seen_nodes.contains(decl)) continue;
             if (decl.data.declName()) |name| {
                 if (seen_in_list.contains(name)) continue;
                 try seen_in_list.put(name, {});
             }
+            try seen_nodes.put(decl, {});
             try decl_list.append(allocator, decl);
         }
         // Separately track which decls the directory `re-exports` to its
diff --git a/tests/expected/issue-0056-diamond-param-impl.exit b/tests/expected/issue-0056-diamond-param-impl.exit
new file mode 100644
index 0000000..573541a
--- /dev/null
+++ b/tests/expected/issue-0056-diamond-param-impl.exit
@@ -0,0 +1 @@
+0
diff --git a/tests/expected/issue-0056-diamond-param-impl.txt b/tests/expected/issue-0056-diamond-param-impl.txt
new file mode 100644
index 0000000..7f8f011
--- /dev/null
+++ b/tests/expected/issue-0056-diamond-param-impl.txt
@@ -0,0 +1 @@
+7