diff --git a/install b/install
index 9c283df..89d5247 100755
--- a/install
+++ b/install
@@ -98,7 +98,7 @@ check_afp_entries() {
         if curl -s -L $AFP_URL | tar zxf - -C .afp $extract; then 
             for e in $missing; do 
                 echo "           Registering $e in $ISABELLE_HOME_USER/ROOTS"
-                echo "$PWD/.afp/afp-2018-08-14/thys/$e" >> $ISABELLE_HOME_USER/ROOTS 
+		grep -q $PWD/.afp/afp-2018-08-14/thys/$e || echo "$PWD/.afp/afp-2018-08-14/thys/$e" >> $ISABELLE_HOME_USER/ROOTS 
             done
             echo "           AFP installation successful."
         else