diff --git a/sftp/hooks/init b/sftp/hooks/init
index 48094b0..a26e3ad 100755
--- a/sftp/hooks/init
+++ b/sftp/hooks/init
@@ -19,9 +19,10 @@ set -e
 
 service_def=$(get_compose_service_def "$SERVICE_NAME")
 
+existing_gids=($(cached_cmd_on_base_image "$SERVICE_NAME" getent group | cut -f 1,3 -d :))
 users_def=$(echo "$service_def" | shyaml get-value options.users 2>/dev/null) || true
 
-build_script="$(make_build_script "$users_def")" || exit 1
+build_script="$(make_build_script "$users_def" "${existing_gids[@]}")" || exit 1
 echo "build-script:" >&2
 echo "$build_script" | prefix "  $GRAY|$NORMAL " >&2
 docker_update "$SERVICE_NAME" "$build_script" -v "$SERVICE_DATASTORE/home":"/home" || exit 1
diff --git a/sftp/lib/common b/sftp/lib/common
index 79f3a24..7610908 100644
--- a/sftp/lib/common
+++ b/sftp/lib/common
@@ -9,22 +9,33 @@ make_build_script() {
         touch "$cache_file" || return 1
         return 0
     fi
+    ## remaining arguments are the group_label:gid list that should be
+    ## avoided for creation (because already existent ?)
+    shift
 
-    local users_def="$1" \
-          code fixed_groups_code groups_code volume_keys \
-          created_groups first_group
+    local code fixed_groups_code groups_code \
+          created_groups first_group gids
 
     if [ -z "$users_def" ]; then
         return 0
     fi
-
     e "set -eux"$'\n'
     code=""
     fixed_groups_code=""
     groups_code=""
-    volume_keys=()
-
+    gids=()
     declare -A created_groups
+    for group in "$@"; do
+        if [[ "$group" != *":"* ]]; then
+            err "Invalid group specification given from script"
+            return 1
+        fi
+        gid=${group##*:}
+        gids+=("$gid")
+        group_label=${group%%:*}
+        created_groups[$group_label]="$gid"
+    done
+
     while read-0 user user_def; do
 
         code+="mkdir -p \"/home/$user\""$'\n'
@@ -34,28 +45,51 @@ make_build_script() {
         ##
 
         first_group=
-        groups=()
         first=1
         while read-0 group; do
-            [ "${created_groups[$group]}" ] && continue
+            gid="unspecified"
             if [[ "$group" == *":"* ]]; then
                 gid=${group##*:}
-                group=${group%%:*}
-                fixed_groups_code+="addgroup -g \"$gid\" \"$group\""$'\n'
+                if ! [[ "$gid" =~ ^[0-9]+$ ]]; then
+                    err "gid part (after ':') should be numerical not '$gid'."
+                    return 1
+                fi
+                group_label=${group%%:*}
             else
-                groups_code+="addgroup \"$group\""$'\n'
+                group_label="$group"
             fi
-            created_groups[$group]=1
+            if [ "${created_groups[$group_label]}" ]; then  ## already met this group
+                ## double-check we did not give conflicting gid information
+                if [[ "${created_groups[$group_label]}" != "$gid" ]]; then
+                    err "Conflicting setup: using existing group '$group_label' for user '$user'" \
+                        "with different GIDs specification" \
+                        "(previous was ${created_groups[$group_label]}, current one is $gid)."
+                    return 1
+                fi
+            else
+                if [[ "$gid" != unspecified ]]; then
+                    ## check not already used
+                    if [[ " ${gids[*]} " == *" $gid "* ]]; then
+                        err "gid $gid is alreay used and is not available."
+                        return 1
+                    fi
+                    fixed_groups_code+="addgroup -g \"$gid\" \"$group_label\""$'\n'
+                else
+                    groups_code+="addgroup \"$group\""$'\n'
+                fi
+                created_groups[$group_label]="$gid"
+            fi
+            echo "First: $first" >&2
+            echo "First: $group_label" >&2
             if [ "$first" ]; then
-                first_group="$group"
+                echo "FG1: $first_group" >&2
+                first_group="$group_label"
                 first=
             else
-                remaining_groups+=("$group")
+                remaining_groups+=("$group_label")
             fi
-            groups+=("$group")
         done < <(echo "$user_def" | shyaml get-values-0 groups 2>/dev/null)
 
-
         ##
         ## User create commands
         ##