diff --git a/regression/verilog/modules/input_and_output.desc b/regression/verilog/modules/input_and_output.desc
index 7f7b3008f..6294e694d 100644
--- a/regression/verilog/modules/input_and_output.desc
+++ b/regression/verilog/modules/input_and_output.desc
@@ -1,8 +1,8 @@
-KNOWNBUG
+CORE
 input_and_output.v
 
+^file .* line 4: port `x' is alrady declared$
 ^EXIT=2$
 ^SIGNAL=0$
 --
 --
-This should be errored, as some_var must not be both input and output.
diff --git a/src/verilog/verilog_elaborate.cpp b/src/verilog/verilog_elaborate.cpp
index 85382c6cf..d26214121 100644
--- a/src/verilog/verilog_elaborate.cpp
+++ b/src/verilog/verilog_elaborate.cpp
@@ -286,14 +286,23 @@ void verilog_typecheckt::collect_symbols(const verilog_declt &decl)
         {
           symbolt &osymbol = *result;
 
+          // 1800-2017 23.2.2.1
+          // "Once a name is used in a port declaration, it shall not be declared
+          // again in another port declaration"
+          if(osymbol.is_input || osymbol.is_output)
+          {
+            throw errort().with_location(declarator.source_location())
+              << "port `" << symbol.base_name << "' is alrady declared";
+          }
+
           if(symbol.type != osymbol.type)
           {
             if(get_width(symbol.type) > get_width(osymbol.type))
               osymbol.type = symbol.type;
           }
 
-          osymbol.is_input = symbol.is_input || osymbol.is_input;
-          osymbol.is_output = symbol.is_output || osymbol.is_output;
+          osymbol.is_input = symbol.is_input;
+          osymbol.is_output = symbol.is_output;
           osymbol.is_state_var = symbol.is_state_var || osymbol.is_state_var;
 
           // a register can't be an input as well