diff --git a/libraries/AP_Param/AP_Param.cpp b/libraries/AP_Param/AP_Param.cpp
index f9aed87658..6e2afd70ff 100644
--- a/libraries/AP_Param/AP_Param.cpp
+++ b/libraries/AP_Param/AP_Param.cpp
@@ -117,10 +117,20 @@ bool AP_Param::check_group_info(const struct AP_Param::GroupInfo *  group_info,
                                 uint8_t                             prefix_length)
 {
     uint8_t type;
-    int8_t max_idx = -1;
+    uint64_t used_mask = 0;
     for (uint8_t i=0;
          (type=group_info[i].type) != AP_PARAM_NONE;
          i++) {
+        uint8_t idx = group_info[i].idx;
+        if (idx >= (1<<_group_level_shift)) {
+            Debug("idx too large (%u) in %s", idx, group_info[i].name);
+            return false;
+        }
+        if (used_mask & (1ULL<<idx)) {
+            Debug("Duplicate group idx %u for %s", idx, group_info[i].name);
+            return false;
+        }
+        used_mask |= (1ULL<<idx);
         if (type == AP_PARAM_GROUP) {
             // a nested group
             const struct GroupInfo *ginfo = group_info[i].group_info;
@@ -134,16 +144,6 @@ bool AP_Param::check_group_info(const struct AP_Param::GroupInfo *  group_info,
             }
             continue;
         }
-        uint8_t idx = group_info[i].idx;
-        if (idx >= (1<<_group_level_shift)) {
-            Debug("idx too large (%u) in %s", idx, group_info[i].name);
-            return false;
-        }
-        if ((int8_t)idx <= max_idx) {
-            Debug("indexes must be in increasing order in %s", group_info[i].name);
-            return false;
-        }
-        max_idx = (int8_t)idx;
         uint8_t size = type_size((enum ap_var_type)type);
         if (size == 0) {
             Debug("invalid type in %s", group_info[i].name);