diff --git a/lib/taski/progress/layout/tree/structure.rb b/lib/taski/progress/layout/tree/structure.rb
index 955bec8..8ee270e 100644
--- a/lib/taski/progress/layout/tree/structure.rb
+++ b/lib/taski/progress/layout/tree/structure.rb
@@ -24,8 +24,7 @@ def render_tree
 
           def init_tree_structure
             @tree_nodes = {}
-            @node_depths = {}
-            @node_is_last = {}
+            @node_prefixes = {}
           end
 
           def build_ready_tree
@@ -64,9 +63,17 @@ def register_tree_nodes(node, depth:, is_last:, ancestors_last:)
 
             task_class = node[:task_class]
             @tasks[task_class] ||= new_task_progress
-            @tree_nodes[task_class] = node
-            @node_depths[task_class] = depth
-            @node_is_last[task_class] = {is_last: is_last, ancestors_last: ancestors_last.dup}
+            @tree_nodes[task_class] ||= node
+
+            # A shared dependency is registered once per occurrence in the
+            # tree. The prefix belongs to the OCCURRENCE (stored on the node),
+            # not to the task class — a class-keyed prefix would be clobbered
+            # by the last occurrence and corrupt every earlier row. The
+            # class-keyed map keeps only the FIRST occurrence, as the one
+            # canonical prefix for per-event output lines.
+            prefix = compute_tree_prefix(depth: depth, is_last: is_last, ancestors_last: ancestors_last)
+            node[:prefix] = prefix
+            @node_prefixes[task_class] ||= prefix
 
             children = node[:children]
             children.each_with_index do |child, index|
@@ -88,9 +95,8 @@ def build_tree_lines
           def build_node_lines(node, lines)
             return unless node
 
-            task_class = node[:task_class]
-            prefix = build_tree_prefix(task_class)
-            content = build_task_content(task_class)
+            prefix = node[:prefix] || ""
+            content = build_task_content(node[:task_class])
             lines << "#{prefix}#{content}"
 
             node[:children].each do |child|
@@ -116,15 +122,14 @@ def build_task_content(task_class)
             end
           end
 
+          # Canonical (first-occurrence) prefix for a task class, used by
+          # per-event output lines in Tree::Event.
           def build_tree_prefix(task_class)
-            depth = @node_depths[task_class]
-            return "" if depth.nil? || depth == 0
-
-            last_info = @node_is_last[task_class]
-            return "" unless last_info
+            @node_prefixes[task_class] || ""
+          end
 
-            ancestors_last = last_info[:ancestors_last]
-            is_last = last_info[:is_last]
+          def compute_tree_prefix(depth:, is_last:, ancestors_last:)
+            return "" if depth.zero?
 
             prefix = ""
             # Skip the first ancestor (root) since root has no visual prefix
@@ -132,8 +137,7 @@ def build_tree_prefix(task_class)
               prefix += ancestor_is_last ? SPACE : VERTICAL
             end
 
-            prefix += is_last ? LAST_BRANCH : BRANCH
-            prefix
+            prefix + (is_last ? LAST_BRANCH : BRANCH)
           end
         end
       end
diff --git a/test/test_layout_tree_event.rb b/test/test_layout_tree_event.rb
index 9b9525d..28a42b8 100644
--- a/test/test_layout_tree_event.rb
+++ b/test/test_layout_tree_event.rb
@@ -283,6 +283,65 @@ def test_root_task_has_no_prefix
     refute_includes @output.string, "└── ⠋ RootTask"
   end
 
+  # A shared dependency (diamond: Root -> Left -> Shared, Root -> Shared)
+  # appears once per occurrence in the rendered tree — each occurrence with
+  # the prefix of ITS OWN position. Prefix data must be per-node, not keyed
+  # by task class (a later registration must not clobber an earlier one).
+  def test_render_tree_with_shared_dependency_renders_each_occurrence_with_its_own_prefix
+    shared = stub_task_class("SharedTask")
+    left = stub_task_class_with_deps("LeftTask", [shared])
+    root = stub_task_class_with_deps("RootTask", [left, shared])
+
+    ctx = mock_execution_facade(root_task_class: root)
+    @layout.context = ctx
+    @layout.on_ready
+
+    assert_equal [
+      "○ RootTask",
+      "├── ○ LeftTask",
+      "│   └── ○ SharedTask",
+      "└── ○ SharedTask"
+    ], @layout.render_tree.lines.map(&:chomp)
+  end
+
+  # Event-mode per-event lines use one canonical prefix for a shared task:
+  # its first occurrence in the tree (the position it is first printed at).
+  def test_event_output_for_shared_dependency_uses_first_occurrence_prefix
+    shared = stub_task_class("SharedTask")
+    left = stub_task_class_with_deps("LeftTask", [shared])
+    root = stub_task_class_with_deps("RootTask", [left, shared])
+
+    ctx = mock_execution_facade(root_task_class: root)
+    @layout.context = ctx
+    @layout.on_ready
+    @layout.on_start
+    @layout.on_task_updated(shared, previous_state: :pending, current_state: :running, phase: :run, timestamp: Time.now)
+    @layout.on_stop
+
+    assert_includes @output.string, "│   └── ⠋ SharedTask"
+  end
+
+  # A circular graph renders the cycle as a childless leaf occurrence. When
+  # the ROOT class reappears (Root -> A -> Root), the class-keyed node map
+  # must keep the ROOT occurrence (the first registration), not the circular
+  # leaf — otherwise build_tree_lines walks the leaf and the whole tree
+  # collapses to a single orphan line.
+  def test_render_tree_with_root_cycle_renders_full_tree_from_root_occurrence
+    a = stub_task_class("ATask")
+    root = stub_task_class_with_deps("RootTask", [a])
+    a.define_singleton_method(:cached_dependencies) { [root] }
+
+    ctx = mock_execution_facade(root_task_class: root)
+    @layout.context = ctx
+    @layout.on_ready
+
+    assert_equal [
+      "○ RootTask",
+      "└── ○ ATask",
+      "    └── ○ RootTask"
+    ], @layout.render_tree.lines.map(&:chomp)
+  end
+
   private
 
   def stub_task_class(name)