Fix bug that "trace line" does not work after stopping trace once

Author: ono-max

lib/debug/session.rb

@@ -1524,7 +1524,7 @@ def add_iseq_breakpoint iseq, **kw
     # tracers
 
     def add_tracer tracer
-      if @tracers.has_key? tracer.key
+      if @tracers[tracer.key]&.enabled?
         tracer.disable
         @ui.puts "Duplicated tracer: #{tracer}"
       else

lib/debug/tracer.rb

@@ -54,6 +54,10 @@ def disable
       @tracer.disable
     end
 
+    def enabled?
+      @tracer.enabled?
+    end
+
     def description
       nil
     end

test/console/trace_test.rb

@@ -532,5 +532,31 @@ def test_tracer_prints_correct_method_receiving_messages
         end
       end
     end
+
+    class TraceOnAfterStoppingOnceTest < ConsoleTestCase
+      def program
+        <<~RUBY
+          1| a=1
+          2|
+          3| b=1
+          4|
+          5| c=1
+          6| p a
+        RUBY
+      end
+  
+      def test_1656237686
+        debug_code(program) do
+          type 'trace line'
+          type 'trace off'
+          type 'trace line'
+          type 'b 5'
+          type 'c'
+          assert_line_num 5
+          assert_line_text(/DEBUGGER \(trace\/line\)/)
+          type 'kill!'
+        end
+      end
+    end
   end
 end