diff --git a/configure b/configure
index 3d7edab6..f0a3044a 100755
--- a/configure
+++ b/configure
@@ -70,8 +70,26 @@ assign_opt() {
   eval ${1#--}=\$2  # no risk of IFS/glob in [OPT]NAME
 }
 
+# succeed if $1 doesn't IFS-split funny (globs, spaces, ...)
+good_split() {
+  set -- "$1" $1
+  test $# = 2 && test "$1" = "$2"
+}
+
+# $1: NAME[=VALUE]  succeed if confvars has NAME or NAME=* element
+confvars_has() {
+  ! case " $confvars " in *" ${1%%=*} "* | *" ${1%%=*}="*)
+    false
+  esac
+}
+
+# [multiple] NAME or NAME=VAL
 confvars_set() {
-  confvars="$confvars $*"
+  for cv; do
+    good_split "$cv" || { echo "configure: ERROR: bad config '$cv'"; exit 1; }
+    confvars_has "$cv" && echo "configure: WARNING: duplicate config '$cv'"
+    confvars="$confvars $cv"
+  done
 }
 
 for opt do
@@ -221,9 +239,9 @@ default() # set variable unless already set and not empty
     test -n "$2" && eval : \${$1:=\$2}  # ': ${foo:=$2}'
 }
 
-default_conf() # add to confvars unless already present
+default_conf() # add one config to confvars unless already present
 {
-    test "${confvars%${1%=*}*}" = "${confvars}" && confvars_set "$1"
+    confvars_has "$1" || confvars_set "$1"
 }
 
 if test -z "${source_path#.}" ; then