diff --git a/driver/iverilog.man.in b/driver/iverilog.man.in
index 47ebf4669..3861690d2 100644
--- a/driver/iverilog.man.in
+++ b/driver/iverilog.man.in
@@ -316,8 +316,8 @@ after a \fB\-Wall\fP argument to suppress isolated warning types.
 
 .TP 8
 .B all
-This enables the anachronisms, implicit, portbind, select\-range,
-timescale, and sensitivity\-entire\-array warning categories.
+This enables the anachronisms, implicit, macro-redefinition, portbind,
+select\-range, timescale, and sensitivity\-entire\-array warning categories.
 
 .TP 8
 .B anachronisms
@@ -330,6 +330,10 @@ This enables warnings for creation of implicit declarations. For
 example, if a scalar wire X is used but not declared in the Verilog
 source, this will print a warning at its first use.
 
+.TP 8
+.B macro-redefinition
+This enables preprocessor warnings when a macro is being redefined.
+
 .TP 8
 .B portbind
 This enables warnings for ports of module instantiations that are not
diff --git a/driver/main.c b/driver/main.c
index fa038bfe6..d12c40fe1 100644
--- a/driver/main.c
+++ b/driver/main.c
@@ -138,7 +138,7 @@ int gen_std_include = 1;
    of the include list. */
 int gen_relative_include = 0;
 
-char warning_flags[16] = "n";
+char warning_flags[17] = "n";
 
 int separate_compilation_flag = 0;
 
@@ -346,10 +346,14 @@ static int t_version_only(void)
 
 static void build_preprocess_command(int e_flag)
 {
-      snprintf(tmp, sizeof tmp, "%s%civlpp %s%s -F\"%s\" -f\"%s\" -p\"%s\"%s",
-	       ivlpp_dir, sep, verbose_flag?" -v":"",
-	       e_flag?"":" -L", defines_path, source_path,
-	       compiled_defines_path, e_flag?"":" | ");
+      snprintf(tmp, sizeof tmp, "%s%civlpp %s%s%s -F\"%s\" -f\"%s\" -p\"%s\"%s",
+	       ivlpp_dir, sep,
+               verbose_flag ? " -v" : "",
+	       e_flag ? "" : " -L",
+               strchr(warning_flags, 'r') ? " -Wredef " : "",
+               defines_path, source_path,
+	       compiled_defines_path,
+	       e_flag ? "" : " | ");
 }
 
 static int t_preprocess_only(void)
@@ -520,6 +524,10 @@ static void process_warning_switch(const char*name)
 	    process_warning_switch("select-range");
 	    process_warning_switch("timescale");
 	    process_warning_switch("sensitivity-entire-array");
+	    process_warning_switch("macro-redefinition");
+      } else if (strcmp(name,"macro-redefinition") == 0) {
+          if (! strchr(warning_flags, 'r'))
+ 		  strcat(warning_flags, "r");
       } else if (strcmp(name,"anachronisms") == 0) {
 	    if (! strchr(warning_flags, 'n'))
 		  strcat(warning_flags, "n");
@@ -558,6 +566,12 @@ static void process_warning_switch(const char*name)
 		  cp[0] = cp[1];
 		  cp += 1;
 	    }
+      } else if (strcmp(name,"no-macro-redefinition") == 0) {
+        char*cp = strchr(warning_flags, 'r');
+  	    if (cp) while (*cp) {
+  		  cp[0] = cp[1];
+  		  cp += 1;
+  	    }
       } else if (strcmp(name,"no-floating-nets") == 0) {
 	    char*cp = strchr(warning_flags, 'f');
 	    if (cp) while (*cp) {
@@ -1306,8 +1320,11 @@ int main(int argc, char **argv)
 	/* Write the preprocessor command needed to preprocess a
 	   single file. This may be used to preprocess library
 	   files. */
-      fprintf(iconfig_file, "ivlpp:%s%civlpp -L -F\"%s\" -P\"%s\"\n",
-	      ivlpp_dir, sep, defines_path, compiled_defines_path);
+      fprintf(iconfig_file, "ivlpp:%s%civlpp %s -L -F\"%s\" -P\"%s\"\n",
+	      ivlpp_dir, sep,
+              strchr(warning_flags, 'r') ? "-Wredef" : "",
+              defines_path, compiled_defines_path
+      );
 
 	/* Done writing to the iconfig file. Close it now. */
       fclose(iconfig_file);
diff --git a/ivlpp/globals.h b/ivlpp/globals.h
index 90b4632b9..1c4cee43d 100644
--- a/ivlpp/globals.h
+++ b/ivlpp/globals.h
@@ -53,6 +53,8 @@ extern char dep_mode;
 
 extern int verbose_flag;
 
+extern int warn_redef;
+
 /* This is the entry to the lexer. */
 extern int yylex(void);
 
diff --git a/ivlpp/lexor.lex b/ivlpp/lexor.lex
index c81af7397..b8ccb6296 100644
--- a/ivlpp/lexor.lex
+++ b/ivlpp/lexor.lex
@@ -858,6 +858,19 @@ void define_macro(const char* name, const char* value, int keyword, int argc)
 {
     int idx;
     struct define_t* def;
+    struct define_t* prev;
+
+    /* Verilog spec has a very nasty system of macros jumping from
+     * file to file, resulting in a global macro scope. We abosolutely
+     * MUST track macro redefinitions and warn user about them.
+     */
+
+    prev = def_lookup(name);
+    if (prev && warn_redef) {
+        emit_pathline(istack);
+        fprintf(stderr, "warning: redefinition of macro %s from value '%s' to '%s'\n",
+            name, prev->value, value);
+    }
 
     def = malloc(sizeof(struct define_t));
     def->name = strdup(name);
diff --git a/ivlpp/main.c b/ivlpp/main.c
index 7ec80cb86..b4909b647 100644
--- a/ivlpp/main.c
+++ b/ivlpp/main.c
@@ -98,6 +98,9 @@ int line_direct_flag = 0;
 unsigned error_count = 0;
 FILE *depend_file = NULL;
 
+/* Should we warn about macro redefinitions? */
+int warn_redef = 0;
+
 static int flist_read_flags(const char*path)
 {
       char line_buf[2048];
@@ -282,7 +285,7 @@ int main(int argc, char*argv[])
       include_dir[0] = 0;  /* 0 is reserved for the current files path. */
       include_dir[1] = strdup(".");
 
-      while ((opt=getopt(argc, argv, "F:f:K:Lo:p:P:vV")) != EOF) switch (opt) {
+      while ((opt=getopt(argc, argv, "F:f:K:Lo:p:P:vVW:")) != EOF) switch (opt) {
 
 	  case 'F':
 	    flist_read_flags(optarg);
@@ -336,6 +339,12 @@ int main(int argc, char*argv[])
 		break;
 	  }
 
+	  case 'W': {
+		  if (strcmp(optarg, "redef")==0)
+		  	warn_redef = 1;
+		  break;
+	  }
+
 	  case 'v':
 	    fprintf(stderr, "Icarus Verilog Preprocessor version "
 		    VERSION " (" VERSION_TAG ")\n\n");
@@ -366,7 +375,8 @@ int main(int argc, char*argv[])
 		    "    -p<fil> - Write precompiled defines to <fil>\n"
 		    "    -P<fil> - Read precompiled defines from <fil>\n"
 		    "    -v      - Verbose\n"
-		    "    -V      - Print version information and quit\n",
+		    "    -V      - Print version information and quit\n"
+			"    -W<cat> - Enable extra ivlpp warning category (e.g. redef)\n",
 		    argv[0]);
 	    return flag_errors;
       }