diff --git a/Docs/src/defines.but b/Docs/src/defines.but
index b090cbd6..08c0e82d 100644
--- a/Docs/src/defines.but
+++ b/Docs/src/defines.but
@@ -10,7 +10,7 @@ Define/conditional compilation related commands:
 
 \S1{define} !define
 
-\c ([/date|/utcdate] gflag [value]) | (/math gflag val1 OP val2) | (/file gflag filename.txt)
+\c [/ifndef | /redef] ([/date|/utcdate] gflag [value]) | (/math gflag val1 OP val2) | (/file gflag filename.txt)
 
 This command will add \e{gflag} to the global define list. This will have a similar effect as using the /D switch on the command line (only the define only becomes effective after the !define command).
 
@@ -26,6 +26,7 @@ If \e{/file} is used, the entire text file specified (including whitespace and n
 \c !define /math RESULT 3 + 10
 \c !define /math REST 15 % ${RESULT}
 \c !define /file BUNCHASTUFF somesourcefile.cpp
+\c !define /redef USE_SOMETHING ${RESULT} ;redefine USE_SOMETHING
 
 \S1{undef} !undef
 
diff --git a/Source/script.cpp b/Source/script.cpp
index 5b9d271d..070fc8ee 100644
--- a/Source/script.cpp
+++ b/Source/script.cpp
@@ -2799,6 +2799,21 @@ int CEXEBuild::doCommand(int which_token, LineParser &line)
       GrowBuf file_buf;
       TCHAR datebuf[256];
       TCHAR mathbuf[256];
+      int dupemode=0;
+
+
+      if (!_tcsicmp(define,_T("/ifndef")))
+        dupemode=1;
+      else if (!_tcsicmp(define,_T("/redef")))
+        dupemode=2;
+
+      if (dupemode!=0)
+      {
+        line.eattoken();
+        define=line.gettoken_str(1);
+        if (dupemode==1 && definedlist.find(define))return PS_OK;
+      }
+
 
       if (!_tcsicmp(define,_T("/date")) || !_tcsicmp(define,_T("/utcdate"))) {
         if (line.getnumtokens()!=4) PRINTHELP()
@@ -2903,6 +2918,7 @@ int CEXEBuild::doCommand(int which_token, LineParser &line)
         value=line.gettoken_str(2);
       }
 
+      if (dupemode==2)definedlist.del(define);
       if (definedlist.add(define,value))
       {
         ERROR_MSG(_T("!define: \"%s\" already defined!\n"),define);
diff --git a/Source/tokens.cpp b/Source/tokens.cpp
index f4a7f2ea..2764dc47 100644
--- a/Source/tokens.cpp
+++ b/Source/tokens.cpp
@@ -246,7 +246,7 @@ static tokenType tokenlist[TOK__LAST] =
 {TOK_P_IFDEF,_T("!ifdef"),1,-1,_T("symbol [| symbol2 [& symbol3 [...]]]"),TP_ALL},
 {TOK_P_IFNDEF,_T("!ifndef"),1,-1,_T("symbol [| symbol2 [& symbol3 [...]]]"),TP_ALL},
 {TOK_P_ENDIF,_T("!endif"),0,0,_T(""),TP_ALL},
-{TOK_P_DEFINE,_T("!define"),1,4,_T("([/date|/utcdate] symbol [value]) | (/file symbol filename) | (/math symbol val1 OP val2)\n    OP=(+ - * / % & | ^)"),TP_ALL},
+{TOK_P_DEFINE,_T("!define"),1,5,_T("[/ifndef | /redef] ([/date|/utcdate] symbol [value]) | (/file symbol filename) | (/math symbol val1 OP val2)\n    OP=(+ - * / % & | ^)"),TP_ALL},
 {TOK_P_UNDEF,_T("!undef"),1,1,_T("symbol [value]"),TP_ALL},
 {TOK_P_ELSE,_T("!else"),0,-1,_T("[if[macro][n][def] ...]"),TP_ALL},
 {TOK_P_ECHO,_T("!echo"),1,0,_T("message"),TP_ALL},