Update btrees.nim (#14916)

compiler/btrees.nim

@@ -79,6 +79,9 @@ proc insert[Key, Val](h: Node[Key, Val], key: Key, val: Val): Node[Key, Val] =
   var j = 0
   if not h.isInternal:
     while j < h.entries:
+      if eq(key, h.keys[j]):
+        h.vals[j] = val
+        return
       if less(key, h.keys[j]): break
       inc j
     for i in countdown(h.entries, j+1):