[neighorch]: Return false when the next hop IP exists before adding

It is possible that due to events reordering, a different next hop
entry with same IP will be inserted before the previous next hop
entry gets removed. Thus, return false here to prevent the new entry
from replacing the old one automatically.

Signed-off-by: Shu0T1an ChenG <shuche@microsoft.com>

orchagent/neighorch.cpp

@@ -30,6 +30,13 @@ bool NeighOrch::addNextHop(IpAddress ipAddress, string alias)
 {
     SWSS_LOG_ENTER();
 
+    if (hasNextHop(ipAddress))
+    {
+        SWSS_LOG_ERROR("Next hop IP %s already exists",
+                ipAddress.to_string().c_str());
+        return false;
+    }
+
     Port p;
     if (!gPortsOrch->getPort(alias, p))
     {
@@ -38,7 +45,6 @@ bool NeighOrch::addNextHop(IpAddress ipAddress, string alias)
         return false;
     }
 
-    assert(!hasNextHop(ipAddress));
     sai_object_id_t rif_id = m_intfsOrch->getRouterIntfsId(alias);
 
     vector<sai_attribute_t> next_hop_attrs;