Fix accidental duplicaton in environment

	This was evident by typing env several times
	Could cause some programs to misbehave

test.xs

@@ -63,11 +63,15 @@ fn expect-success { ||
 fn expect-failure {
     ! expect-success
 }
+fn match-abs { |result|
+    log2 Absolute matching...
+    ~ `` '' output $^result
+}
 fn match { |result|
     log2 Matching...
     ~ `` '' output *^$^result^*
 }
-fn match_re { |result|
+fn match-re { |result|
     log2 Match_re....
     eval '~ `` '''' output *'^$^result^'*'
 }

var.cxx

@@ -220,6 +220,7 @@ static void mkenv0(Dict::value_type pair) {
 
 extern Vector* mkenv(void) {
 	if (isdirty || rebound) {
+		env.clear();
 		std::for_each(vars.begin(), vars.end(), mkenv0);
 
 		isdirty = false;